Determining Intervals on Increasing, Decreasing Functions

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AP Calculus BC › Determining Intervals on Increasing, Decreasing Functions

Questions 1 - 10
1

If $f'(x)=(x-2)(x+1)$ for all real $x$, on which interval(s) is $f$ increasing?

$(-\infty,-1)\cup(2,\infty)$

$(-1,2)$

$(-\infty,-1)\cup(-1,2)$

$(-\infty,2)$

$(2,\infty)$ only

Explanation

Determining the intervals where a function is increasing or decreasing is a key skill in AP Calculus BC, relying on the first derivative test. The function f is increasing where its derivative f'(x) is positive, which occurs when (x-2)(x+1) > 0. By finding the critical points x = -1 and x = 2, we divide the real line into intervals and test the sign of f'(x) in each: positive in (-∞, -1) and (2, ∞), and negative in (-1, 2). Thus, f is increasing on (-∞, -1) ∪ (2, ∞). A tempting distractor like (-1, 2) fails because that's where f'(x) is negative, indicating the function is actually decreasing there. Always create a sign chart using the roots of the derivative to systematically determine where it is positive or negative.

2

Suppose $h'(x)=\dfrac{(x-4)(x+1)}{x^2+1}$. On which interval(s) is $h$ increasing?

$(4,\infty)$ only

$(-\infty,4)$

$(-\infty,-1)$

$(-1,4)$

$(-\infty,-1)\cup(4,\infty)$

Explanation

Determining the intervals where a function is increasing or decreasing is a key skill in AP Calculus BC, relying on the first derivative test. The function h is increasing where its derivative h'(x) = $(x-4)(x+1)/(x^2$+1) is positive; since the denominator is always positive, the sign matches the numerator. The critical points are x = -1 and x = 4, dividing the line into intervals where the numerator is positive in (-∞, -1) ∪ (4, ∞) and negative in (-1, 4). Thus, h is increasing on (-∞, -1) ∪ (4, ∞). A tempting distractor like (-∞, 4) fails because it includes (-1, 4) where h'(x) is negative, meaning the function decreases there. Always create a sign chart using the roots of the derivative to systematically determine where it is positive or negative.

3

If $f'(x)=\dfrac{1-x}{\sqrt{x}}$ for $x>0$, on which interval(s) is $f$ increasing?

$(0,\infty)$

$(0,1)$

$(-\infty,1)$

$(0,1)\cup(1,\infty)$

$(1,\infty)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in calculus that relies on analyzing the sign of the first derivative. The function f is increasing where f'(x) = (1 - x)/√x > 0 for x > 0. Since √x > 0, this holds when 1 - x > 0 or x < 1, so on (0, 1). At x = 1, f' = 0, and for x > 1, f' < 0. A tempting distractor is choice B (1, ∞), but that's where f' < 0, so f decreases there. In general, to analyze the monotonicity of a function, find the critical points where the derivative is zero or undefined, then test the sign of the derivative in each interval determined by those points.

4

For $f'(x)=-(x-2)^2(x+3)$, on which interval(s) is $f$ increasing?

$(-\infty,2)$

$(2,\infty)$

$(-3,\infty)$

$(-\infty,-3)$

$(-3,2)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in AP Calculus BC that relies on analyzing the first derivative. For f'(x)= -(x-2)²(x+3), roots x=2 (double), x=-3. Negative sign, (x-2)² ≥0, (x+3). Product (x-2)²(x+3) has sign of (x+3), zero at 2 and -3. So - of that: when x>-3, positive then negative f'<0; x<-3, negative then negative f'>0? Wait: (x-2)² always pos except zero, times (x+3): pos*(pos)>0 for x>-3, pos*(neg)<0 for x<-3. Then overall - of that: - (>0) = <0 for x>-3; - (<0)= >0 for x<-3. At double root x=2, zero but sign doesn't change (remains neg). So f'>0 on (-∞,-3), f'<0 on (-3,∞). Thus increasing on (-∞,-3). A tempting distractor like C, (2,∞), fails because f'<0 there. Always create a sign chart for the derivative to systematically determine intervals of increase and decrease.

5

Let $f'(x)=(x+5)(x-1)^2$. On which interval(s) is $f$ decreasing?

$(-5,1)$

$(-5,\infty)$

$(-\infty,-5)$

$(1,\infty)$

$(-\infty,1)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in AP Calculus BC that relies on analyzing the first derivative. Given f'(x)=(x+5)(x-1)², roots x=-5, x=1 (double). (x-1)² ≥0, zero at 1; so sign determined by (x+5), but at double root sign doesn't change. f'>0 when x>-5 (except zero at 1), f'<0 when x<-5. Thus decreasing on (-∞,-5). Wait, no: when x<-5, x+5<0, (x-1)²>0, product <0, decreasing; x>-5, >0, increasing. Yes. A tempting distractor like B, (-5,1), fails because f'>0 there, increasing. Always create a sign chart for the derivative to systematically determine intervals of increase and decrease.

6

Let $f'(x)=\dfrac{(x-1)(x+1)}{(x-2)(x+2)}$. On which interval(s) is $f$ decreasing?

$(-2,2)$

$(-\infty,-2)\cup(-1,1)\cup(2,\infty)$

$(-1,1)$

$(-2,-1)\cup(1,2)$

$(-\infty,-1)\cup(1,\infty)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in calculus that relies on analyzing the sign of the first derivative. The function f is decreasing where f'(x) = (x² - 1)/(x² - 4) < 0. This occurs when numerator and denominator have opposite signs, specifically (-2, -1) ∪ (1, 2). Sign analysis confirms. A tempting distractor is choice A (-∞, -2) ∪ (-1, 1) ∪ (2, ∞), but it includes regions where f' > 0. In general, to analyze the monotonicity of a function, find the critical points where the derivative is zero or undefined, then test the sign of the derivative in each interval determined by those points.

7

Given $f'(x)=-(x-3)^2(x+2)$, on which interval is $f$ decreasing?

$(-\infty,3)$

$(-2,\infty)$

$(3,\infty)$

$(-\infty,-2)$

$(-\infty,-2)$ and $(3,\infty)$

Explanation

This question tests the skill of determining intervals where a function is decreasing by analyzing the sign of its first derivative. The derivative f'(x) = $-(x-3)^2$(x+2) indicates that the function f(x) is decreasing where f'(x) < 0 and increasing where f'(x) > 0. The critical points are x = -2 and x = 3, dividing the real line into intervals: (-∞, -2), (-2, 3), and (3, ∞), with f'(x) = 0 at x = 3 but not changing sign there. Sign analysis shows f'(x) > 0 in (-∞, -2) and f'(x) < 0 in (-2, ∞), including through x = 3 where it touches zero but remains decreasing overall. A tempting distractor is choice E, (-∞, -2) and (3, ∞), but (-∞, -2) is where f is increasing, not decreasing. A transferable sign-analysis strategy is to identify critical points, test intervals with representative points, and determine where the derivative is positive for increasing behavior or negative for decreasing behavior.

8

Given $f'(x)=\sqrt{x}-2$ with domain $x\ge0$, on which interval(s) is $f$ decreasing?

$[0,\infty)$

$(0,4]$

$(0,4)$

$[0,4)$

$(4,\infty)$

Explanation

This problem tests finding decreasing intervals with a radical derivative. The derivative f'(x) = √x - 2 equals zero when √x = 2, which gives x = 4. For f to be decreasing, we need f'(x) < 0, which means √x < 2. Since √x is an increasing function and the domain is x ≥ 0, we have √x < 2 precisely when 0 ≤ x < 4. Therefore, f is decreasing on [0, 4). A common mistake is writing (0, 4) and excluding x = 0, but since f'(0) = -2 < 0, the function is indeed decreasing at x = 0. When dealing with derivatives involving radicals, solve the inequality algebraically and remember to check endpoint behavior.

9

If $f'(x)=\dfrac{x+2}{(x-3)^2}$, on which interval(s) is $f$ increasing?

$(-\infty,-2)$

$(-\infty,-2)\cup(-2,3)$

$(-2,3)\cup(3,\infty)$

$(-\infty,3)$

$(3,\infty)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in AP Calculus BC that relies on analyzing the first derivative. For f'(x)=(x+2)/(x-3)², critical point x=-2 (num zero), x=3 undefined (den zero). Denominator always positive except at 3. Sign of f' same as (x+2). So f'>0 when x>-2, f'<0 when x<-2, but excluding 3. Since at 3 undefined, intervals (-∞,-2), (-2,3), (3,∞). f'<0 on (-∞,-2), >0 on (-2,3)∪(3,∞). Yes, increasing on (-2,3)∪(3,∞). A tempting distractor like A, (-∞,-2), fails because that's decreasing. Always create a sign chart for the derivative to systematically determine intervals of increase and decrease.

10

Let $f'(x)=x(x-6)(x+2)$. On which interval(s) is $f$ increasing?

$(-\infty,-2)\cup(0,6)$

$(-2,0)\cup(6,\infty)$

$(0,\infty)$

$(-2,6)$

$(-\infty,0)\cup(6,\infty)$

Explanation

Determining the intervals on which a function is increasing or decreasing is a key skill in AP Calculus BC that relies on analyzing the first derivative. Given f'(x)=x(x-6)(x+2), roots at 0,6,-2. Ordered: -2,0,6. Sign changes at each, cubic with positive leading coefficient. f'>0 in (-2,0) and (6,∞), f'<0 in (-∞,-2) and (0,6). Yes. A tempting distractor like D, (-2,6), fails because it combines increasing and decreasing intervals. Always create a sign chart for the derivative to systematically determine intervals of increase and decrease.

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