Recognize Constant Rate Changes
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Algebra 2 › Recognize Constant Rate Changes
Two tables describe relationships between $x$ and $y$ at equal $x$-intervals.
Table 1: x: 0, 1, 2, 3; y: 6, 9, 12, 15
Table 2: x: 0, 1, 2, 3; y: 2, 4, 8, 16
Which statement is correct?
Neither table shows constant rate of change because both have changing ratios $y/x$.
Only Table 1 shows constant rate of change because its first differences are constant, so it is linear.
Only Table 2 shows constant rate of change because it doubles each time.
Both tables show constant rate of change because $y$ increases in both.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! Table 1 has Δy=3,3,3 constant; Table 2 has ratios 2,2,2 constant but differences 2,4,8 varying. Choice C correctly identifies only Table 1 has constant rate because its first differences are equal, making it linear. Choice B confuses constant ratio (exponential) with constant rate—Table 2 is exponential, not linear! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
A phone plan charges a $\$15$ monthly fee plus $\$0.10$ per text message. Let $C$ be the total cost (in dollars) for $x$ text messages.
Is the rate of change of $C$ with respect to $x$ constant? If so, what is it and what type of function models the situation?
Yes; constant rate of $15$ dollars per text, so the model is linear.
No; because the cost increases, the rate must increase too.
No; because there is a monthly fee, the rate of change is not constant.
Yes; constant rate of $0.10$ dollars per text, so the model is linear.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! The model C=15+0.10x is linear, so constant rate 0.10. Choice B correctly identifies the constant rate of 0.10 because it's y=mx+b form, modeling linear cost. Choice A wrongly assumes a fixed fee means non-constant rate, but the fee is the b (constant), not affecting the steady per-text rate m! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
Consider the function $f(x) = 4x - 7$.
Is the rate of change constant? If so, what is the constant rate $\Delta f/\Delta x$?
No; because the function has a negative number, the rate changes.
No; because $x$ changes, the rate must change too.
Yes; constant rate of $-7$.
Yes; constant rate of $4$, so it is linear.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! The formula f(x)=4x-7 is in y=mx+b form, so constant rate m=4. Choice A correctly identifies the constant rate of 4 because it's linear, and negatives don't affect rate constancy. A distractor like Choice C might confuse the y-intercept -7 with the rate, but rate is the coefficient of x! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
A runner’s distance $d$ (in meters) after $t$ seconds is shown.
| $t$ (s) | 0 | 2 | 4 | 6 |
|---|---|---|---|---|
| $d$ (m) | 0 | 10 | 20 | 30 |
Is the rate of change constant? If so, what is the constant rate $\Delta d/\Delta t$?
Yes; constant rate of $10$ m/s because distance increases by 10 each row.
Yes; constant rate of $5$ m/s because $\Delta d/\Delta t = 10/2 = 5$ for each interval.
No; because time increases by 2 seconds each step, the rate cannot be constant.
No; because the ratio $d/t$ changes, the rate of change is not constant.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! Δd for Δt=2 are 10,10,10, all equal, so Δd/Δt=5 constantly. Choice A correctly identifies the constant rate of 5 because the ratios are equal over equal intervals. Choice C mistakes Δd=10 for the rate, but divide by Δt=2 to get the actual rate 5—always compute Δy/Δx! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
A tank is being filled. The volume of water $V$ (in liters) is recorded every minute $t$.
| $t$ (min) | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| $V$ (L) | 10 | 13 | 17 | 22 | 28 |
Calculate the rate of change over each 1-minute interval. Is the rate of change constant?
Yes; because time increases by 1 each row, the rate must be constant.
No; but only the first interval matters, and it is 3 L/min so the rate is constant at 3 L/min.
Yes; the increases are 3, 4, 5, 6 so the constant rate is 4.5 L/min.
No; $\Delta V$ values (3, 4, 5, 6) are not all equal, so the rate is non-constant (nonlinear).
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! Here, ΔV for Δt=1 are 3,4,5,6, which vary, so non-constant rate. Choice B correctly identifies the non-constant rate because the ΔV values are not equal, indicating nonlinear. A mistake like in Choice A is averaging varying differences, but for constant rate, they must be identical, not averaged! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
Two payment plans are described below.
Plan 1: You pay a $\$10$ sign-up fee plus $$5$ per month.
Plan 2: You pay $\$5$ the first month, $$10$ the second month, $\$20$ the third month, and $$40$ the fourth month.
Which plan has a constant rate of change in total cost with respect to months?
Plan 2 only; it has a constant rate because it multiplies by 2 each month.
Both plans; any repeating pattern is a constant rate of change.
Neither plan; any sign-up fee makes the rate of change non-constant.
Plan 1 only; it increases by a constant $\$5$ per month (linear).
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! Plan 1: total =10+5m, linear with constant Δcost/Δmonth=5; Plan 2: 5,10,20,40..., differences 5,10,20 varying (ratios=2 constant, exponential). Choice A correctly identifies only Plan 1 as constant rate because of equal additions, linear growth. Choice B mistakes Plan 2's constant ratio (multiplying by 2) for constant rate—remember to check differences for linear, ratios for exponential, to distinguish them! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
Which situation has a constant rate of change (and therefore can be modeled by a linear function)?
(a) A tank fills 4 gallons every minute.
(b) The area of a square as its side length increases.
(c) A bacteria population doubles every hour.
Only (b)
(a) and (c)
Only (c)
Only (a)
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! Situation (a) fills 4 gallons per minute, so constant Δgallons/Δtime=4, linear; (b) area A=s², rate dA/ds=2s varies with s, quadratic; (c) doubles hourly, constant ratio 2 but varying differences (e.g., 1 to 2, +1; 2 to 4, +2), exponential. Choice A correctly identifies only (a) as having constant rate because it's additive growth, equivalent to linear. For (c), a common error is thinking doubling is constant rate, but that's constant ratio—calculate differences to confirm they increase, not stay equal! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
A runner’s total distance $d$ (in miles) after $t$ hours is given by $d=6t+1$.
Is the rate of change constant? If so, what is the constant rate and what type of function is it?
No; the rate depends on $t$ since $t$ is a variable.
Yes; constant rate $6$ miles per hour, and it is linear.
No; because of the $+1$, the rate of change is not constant.
Yes; constant rate $1$ mile per hour, and it is quadratic.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! The formula d=6t+1 is in mx+b form with m=6, so constant rate of 6; for example, from t=0 to 1, Δd=(6*1+1)-(1)=7-1=6; t=1 to 2, (12+1)- (6+1)=13-7=6—constant. Choice A correctly identifies the constant rate of 6 and linear type because it's mx+b form. Choice B incorrectly blames the +1 for non-constancy, but constants affect intercept, not rate—focus on the coefficient of t for the rate! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
Does the function $f(x)=3x^2-6x+1$ have a constant rate of change? Explain using the idea of linear vs. nonlinear functions.
Yes; the coefficient of $x$ is $-6$, so the constant rate of change is $-6$.
No; because $f(0)=1$, the function cannot have a constant rate of change.
No; it is quadratic (not of the form $mx+b$), so the rate of change is non-constant.
Yes; all polynomials have a constant rate of change.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! For f(x)=3x²-6x+1, it's quadratic (degree 2, not 1), so the rate varies; for example, from x=0 to 1, Δf=(3-6+1)-(1)= -2 to -1? Wait, f(0)=1, f(1)=3-6+1=-2, Δf=-3; f(2)=12-12+1=1, Δf from 1 to 2=3—not constant. Choice B correctly identifies it as quadratic and thus non-constant rate because it's not in mx+b form. A distractor like choice A might misread the -6x term as the constant rate, but ignore the x² which makes the rate change—always check the highest degree for linearity! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!
A car’s distance from home is recorded every 2 minutes.
Calculate the rate of change over each interval and determine whether it is constant.
| Time $t$ (min) | 0 | 2 | 4 | 6 |
|---|---|---|---|---|
| Distance $d$ (miles) | 0 | 1 | 4 | 9 |
Yes; the rate is constant at $2$ miles per minute.
No; the rates are $\frac{1}{2}, \frac{3}{2}, \frac{5}{2}$ miles per minute, so the rate is non-constant (nonlinear).
Yes; the differences in distance are $1, 3, 5$, which are increasing by 2, so the rate is constant.
No; because time increases by 2 minutes each time, you cannot compute a rate of change.
Explanation
This question tests your ability to recognize when a relationship has constant rate of change—the defining characteristic of linear functions. Constant rate of change means that for every unit increase in x, y changes by the same amount every time: if Δy/Δx = 5 for one interval and also 5 for every other equal interval, the rate is constant at 5. This constant rate is exactly what makes a function linear (y = mx + b where m is that constant rate). Only linear functions have this property—quadratics, exponentials, and other nonlinear functions have rates that vary at different x-values! For this table, calculate the rates: from t=0 to 2, Δd=1-0=1 and Δt=2, rate=1/2; from t=2 to 4, Δd=4-1=3, rate=3/2; from t=4 to 6, Δd=9-4=5, rate=5/2—these vary, so non-constant rate. Choice B correctly identifies the non-constant rates of 1/2, 3/2, 5/2 because the differences increase, indicating a nonlinear (quadratic) relationship. A distractor like choice C might confuse the increasing differences (1,3,5) as a sign of constancy, but remember, for constant rate, the first differences must be equal, not the second differences (which are constant in quadratics)—focus on first differences for linearity! The three-method constant rate test: METHOD 1 (from table): Calculate Δy/Δx for each pair of consecutive points with equal Δx. All equal? Constant rate. Vary? Non-constant. METHOD 2 (from graph): Is it a straight line? Yes = constant rate. Curved? Non-constant. METHOD 3 (from formula): Is it y = mx + b form? Yes = constant rate m. Any other form (x², $b^x$, etc.)? Non-constant. Pick the method matching your representation! Don't confuse constant RATE with constant RATIO: constant rate (Δy/Δx equal) characterizes linear functions, constant ratio (y₂/y₁ equal) characterizes exponential functions. Check BOTH in a table: if differences are 3, 3, 3 → linear with rate 3. If ratios are 2, 2, 2 → exponential with base 2. If neither constant → some other type. Knowing which pattern to look for prevents confusing linear with exponential growth!