Rate of Change at a Point

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AP Calculus BC › Rate of Change at a Point

Questions 1 - 10
1

Table shows $k(1)=3$, $k(4)=15$, $k(7)=24$. What is the average rate of change on $4,7$?​​

$ \frac{24}{7}$

$ \frac{24-15}{7-4}$

$ \frac{24-3}{7-1}$

$ \frac{15-3}{4-1}$

$ \frac{7-4}{24-15}$

Explanation

This problem requires calculating average rate of change from tabular data over the interval [4,7]. Using the formula: (k(7) - k(4))/(7 - 4) = (24 - 15)/(7 - 4) = 9/3 = 3. Choice C might confuse students who use the endpoints [1,7] instead of the requested interval [4,7]. Remember that average rate of change measures the overall change per unit input between two specific points, creating a secant line slope rather than the instantaneous tangent line slope.

2

If $r(t)=3t^4-2t$, what is the instantaneous rate of change of $r$ at $t=0$?

$-2$

$-1$

$0$

$2$

$1$

Explanation

This question asks for the instantaneous rate of change at t = 0, which requires the derivative. The instantaneous rate gives the slope of the tangent line at that exact point, while average rate would give the slope of a secant line over an interval. For r(t) = 3t⁴ - 2t, we find r'(t) = 12t³ - 2, and r'(0) = 12(0)³ - 2 = 0 - 2 = -2. A student might evaluate r(0) = 0 instead of r'(0), incorrectly choosing answer A. The strategy: instantaneous rate always requires differentiation first, then evaluation at the given point.

3

Given $g(2)=5$ and $g(6)=17$, what is the average rate of change of $g$ on $2,6$?

$\dfrac{17-5}{6-2}$

$12$

$3$

$4$

$\dfrac{17}{6}$

Explanation

This question assesses your understanding of the average rate of change over an interval, a key concept in rates of change. The average rate of change is calculated as the difference in function values divided by the difference in inputs, (g(6) - g(2))/(6 - 2) = 12/4 = 3, representing the secant slope. The instantaneous rate differs as it's the derivative at a point, capturing the rate at an exact moment rather than an average over time. While average gives an overall trend, instantaneous provides local behavior, but here no derivative is needed since it's average. A tempting distractor is choice B, 12, which is the numerator alone without dividing by the interval length, overlooking the rate aspect. To distinguish, note if the query involves an interval [a,b] (average) versus a single point (instantaneous).

4

If $f(x) = x + \frac{1}{x}$, what is the instantaneous rate of change at $x=1$?

$-1$

$1$

$2$

$0$

$$

Explanation

This question assesses the concept of instantaneous rate of change at a point. The average rate of change is the net change in the function divided by the change in the input over an interval. The instantaneous rate of change is the limit of the average rate as the interval approaches zero length around the point. Thus, it is given by the derivative of the function at that point. A tempting distractor is to compute $1 + 1/x^2 = 2$ instead of $1 - 1/x^2$, leading to choice C, but the derivative of $1/x$ is $-1/x^2$. A transferable rate-distinction strategy is to check if the query specifies an interval for average rate or a single point for instantaneous rate.

5

If $f(x) = (1-x)^2$, what is the instantaneous rate of change at $x=1$?

$0$

$12$

$1$

$2$

$-2$

Explanation

This question assesses the concept of instantaneous rate of change at a point. The average rate of change is the net change in the function divided by the change in the input over an interval. The instantaneous rate of change is the limit of the average rate as the interval approaches zero length around the point. Thus, it is given by the derivative of the function at that point. A tempting distractor is to ignore the chain rule, taking derivative as $2$ instead of $-2(1-x)$, giving $2$ as choice C, but it's $0$ at $x=1$. A transferable rate-distinction strategy is to check if the query specifies an interval for average rate or a single point for instantaneous rate.

6

If $f(x) = x^5$, what is the instantaneous rate of change at $x=-1$?

$5$

$-5$

$1$

$12$

$-1$

Explanation

This question assesses the concept of instantaneous rate of change at a point. The average rate of change is the net change in the function divided by the change in the input over an interval. The instantaneous rate of change is the limit of the average rate as the interval approaches zero length around the point. Thus, it is given by the derivative of the function at that point. A tempting distractor is to use $5x^4$ with x=-1 as -5, choice A, but $(-1)^4 = 1$, so positive. A transferable rate-distinction strategy is to check if the query specifies an interval for average rate or a single point for instantaneous rate.

7

For $h(x)=\sqrt{x}$, what is the instantaneous rate of change at $x=9$?

$\dfrac{\sqrt{10}-\sqrt{9}}{10-9}$

$\dfrac{1}{18}$

$\dfrac{1}{6}$

$\dfrac{1}{3}$

$\dfrac{\sqrt{9}-\sqrt{0}}{9-0}$

Explanation

This question assesses your understanding of the instantaneous rate of change, which is the derivative at a point. Average rate of change is the secant slope over an interval, (h(b) - h(a))/(b - a), averaging the rate across that span. Instantaneous rate is the limit of that as the interval approaches zero, equaling h'(x) = 1/(2√x), so at x=9 it's 1/6. This distinction highlights how average smooths variations, while instantaneous captures exact slope. A tempting distractor is choice B, 1/3, perhaps from mistakenly using 1/√x instead of the correct derivative. To distinguish rates, identify if it's over an interval (average) or at one point (instantaneous via derivative).

8

For $f(x)=\frac{1}{x}$, what is the instantaneous rate of change at $x=2$?

$-\frac{1}{4}$

$\frac{1}{2}$

$-\frac{1}{2}$

$-\frac{1}{8}$

$\frac{1}{4}$

Explanation

This problem requires the instantaneous rate of change of f(x) = 1/x at x = 2. Instantaneous rate is found using the derivative at a point, not the average change over an interval. The derivative of 1/x is f'(x) = -1/x². At x = 2: f'(2) = -1/2² = -1/4. The negative sign indicates the function is decreasing at x = 2. Choice B (1/4) might attract students who forget the negative sign when differentiating 1/x. Remember that d/dx(x^(-1)) = -x^(-2) = -1/x².

9

For $q(x)=x^2+1$, what is the average rate of change of $q$ on $1,4$?

$3$

$2$

$4$

$5$

$6$

Explanation

This problem asks for the average rate of change of q(x) = x² + 1 on [1, 4]. Average rate measures the overall change divided by the interval width, unlike instantaneous rate which uses derivatives. We calculate: [q(4) - q(1)]/(4 - 1) = [(16 + 1) - (1 + 1)]/(4 - 1) = [17 - 2]/3 = 15/3 = 5. This represents the constant slope of the secant line from (1, 2) to (4, 17). Choice B (3) might tempt students who make an arithmetic error or use the wrong interval. For average rates, carefully evaluate the function at both endpoints before applying the formula.

10

If $h(t)=\sqrt{t}$, what is the instantaneous rate of change at $t=9$?

$\frac{1}{6}$

$\frac{1}{3}$

$\frac{3}{2}$

$\frac{1}{2}$

$\frac{1}{9}$

Explanation

This problem requires the instantaneous rate of change of h(t) = √t at t = 9. The instantaneous rate is found by taking the derivative and evaluating at the specific point, while average rate would use two points. The derivative is h'(t) = 1/(2√t). At t = 9: h'(9) = 1/(2√9) = 1/(2·3) = 1/6. Choice A (1/3) might tempt students who forget the factor of 2 in the denominator of the square root derivative. When differentiating square roots, remember the power rule gives (1/2)t^(-1/2) = 1/(2√t).

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