Graph Proportional Relationships
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8th Grade Math › Graph Proportional Relationships
A runner’s distance $y$ (in miles) is proportional to time $x$ (in hours) and is modeled by $y=6x$. What does the slope (the 6) represent?
The runner’s distance increases by 1 mile every 6 hours.
The runner runs 6 miles total.
The runner goes 6 miles per hour.
The runner goes 6 hours per mile.
Explanation
This question tests graphing proportional y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). In the equation y=6x, the slope 6 means the distance increases by 6 miles for every 1 hour. The correct choice is B because it accurately interprets the slope as the unit rate of 6 miles per hour. A common error is inverting the units, like in A, saying 6 hours per mile. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A proportional relationship is graphed on the coordinate plane. The line goes through $(0,0)$ and $(2,10)$. What is the unit rate (slope) $k$ in $y=kx$?
$k=\dfrac{10}{2}=5$
$k=10-2=8$
$k=\dfrac{2}{10}=0.2$
$k=2+10=12$
Explanation
This question tests graphing proportional y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). The line through (0,0) and (2,10) has slope rise/run=10/2=5. The correct choice is B because it calculates the slope correctly as 10/2=5. A common error is inverting the slope, like in A, using 2/10=0.2. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
Two proportional relationships are shown: Relationship A is $y=2x$. Relationship B is a line through the origin that passes through the point $(1,7)$. Which relationship has the greater rate of change (steeper line)?
Relationship A, because it is written as an equation and B is not.
Relationship A, because $2>\dfrac{1}{7}$.
They have the same rate because both pass through the origin.
Relationship B, because its slope is $7$ and $7>2$.
Explanation
This question tests graphing proportional y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). Relationship B passes through (0,0) and (1,7), so slope=7, which is greater than A's slope of 2. The correct choice is B because it correctly compares the slopes, identifying B's steeper line. A common error is reversing the comparison, like in A, claiming 2>1/7. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A bike rental shop charges a constant rate. The relationship between hours rented $x$ and total cost $y$ is $y=9x$.
How much does it cost to rent a bike for 4 hours?
$13
$18
$36
$45
Explanation
This question tests graphing proportional relationships y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have the form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). For example, in y=9x, for x=4, y=9*4=36 dollars. The correct cost is $36 because it applies the unit rate of $9 per hour correctly for 4 hours. A common error is misapplying the equation, like adding instead of multiplying or using wrong k. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A line on a coordinate plane is given by the equation $y=7x$.
What does the slope represent in this situation?
The line crosses the $y$-axis at 7.
The line is not proportional because it is too steep.
For every 7 unit increase in $x$, $y$ increases by 1 unit.
For every 1 unit increase in $x$, $y$ increases by 7 units.
Explanation
This question tests graphing proportional relationships y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have the form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). For example, in y=7x, the slope k=7 means y increases by 7 for every 1 unit increase in x. The correct choice explains that for every 1 unit increase in x, y increases by 7 units, properly interpreting the slope as the unit rate. A common error is inverting the slope to say for every 7 units in x, y increases by 1, which reverses the rise over run. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A student says the equation $y=2x+3$ is proportional.
Which statement best explains whether the student is correct?
Correct, because $y$ increases as $x$ increases.
Incorrect, because a proportional relationship must pass through $(0,0)$ and this one does not.
Incorrect, because proportional relationships must have a negative slope.
Correct, because the slope is 2.
Explanation
This question tests graphing proportional relationships y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have the form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). For example, in y=2x+3, when x=0, y=3, so it does not pass through (0,0). The correct explanation is that it is incorrect because proportional relationships must pass through (0,0), and this one has a y-intercept of 3. A common error is claiming it's proportional just because y increases with x or focusing on slope alone. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
Two proportional relationships are shown.
- Relationship A: $y=3x$
- Relationship B: $y=\dfrac{1}{3}x$
Which statement is true?
Relationship B is steeper because $\dfrac{1}{3}>3$.
Relationship A is steeper because its slope (unit rate) is larger.
Neither relationship is proportional because one uses a fraction.
Both graphs have the same steepness because they both pass through the origin.
Explanation
This question tests graphing proportional relationships y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have the form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). For example, Relationship A has k=3, while B has k=1/3, so A is steeper since 3>1/3. The correct statement is that Relationship A is steeper because its slope (unit rate) is larger. A common error is reversing the comparison, like claiming 1/3>3 or ignoring the origin. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A proportional relationship is graphed on a coordinate plane. The line passes through the points $(0,0)$ and $(3,12)$.
What is the constant of proportionality $k$ in $y=kx$?
$k=\dfrac{3}{12}$
$k=12$
$k=4$
$k=9$
Explanation
This question tests graphing proportional relationships y=kx, interpreting slope as unit rate, and comparing relationships from different representations. Proportional relationships have the form y=kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). For example, the line passes through (3,12), so k=12/3=4. The correct answer is k=4 because it is properly calculated as the slope rise/run from (0,0) to (3,12). A common error is inverting to k=3/12 or using x instead of y/x. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
A proportional relationship is graphed on a coordinate plane. The line passes through $(0,0)$ and $(4,20)$. Which equation represents the line?
$y=5x$
$y=\frac{1}{5}x$
$y=4x+20$
$y=20x$
Explanation
This question tests graphing proportional relationships of the form y = kx, interpreting the slope as the unit rate, and comparing relationships from different representations. Proportional relationships have the form y = kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). The line passes through (0,0) and (4,20), so slope k = 20/4 = 5, giving y=5x. The correct choice B properly calculates the slope from the points and forms the equation. A common error is inverting the slope to 1/5 or adding an intercept. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).
Which equation shows a proportional relationship between $x$ and $y$?
$y=\frac{x}{2}-1$
$y=7x$
$y=x^2$
$y=2x+3$
Explanation
This question tests graphing proportional relationships of the form y = kx, interpreting the slope as the unit rate, and comparing relationships from different representations. Proportional relationships have the form y = kx (passes through origin, k is constant rate): graphed as straight line through (0,0) with slope k (rise/run ratio constant), interpreted as unit rate (k miles per hour, k dollars per item, k cups per serving). Comparing: steeper slope or larger k indicates greater rate (y=5x faster than y=2x since 5>2). Among the options, y=7x is proportional, passing through (0,0) with constant rate 7, unlike others with intercepts or non-linearity. The correct choice C identifies the equation without intercepts or exponents, ensuring proportionality. A common error is selecting A or B, which have non-zero intercepts, claiming them proportional. Strategy: (1) check origin (proportional must pass through (0,0)), (2) find slope/rate (from graph: rise/run, from table: y/x for any point, from equation: coefficient of x), (3) compare if multiple (larger k or steeper slope wins), (4) interpret (slope 60 in distance-time means 60 miles per hour). Mistakes: forgetting origin requirement, inverting slope, comparing wrong values (using y-intercept when proportional has none).