Determining Intervals on Increasing, Decreasing Functions
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AP Calculus BC › Determining Intervals on Increasing, Decreasing Functions
If $f'(x)=\dfrac{1-x}{\sqrt{x}}$ for $x>0$, on which interval(s) is $f$ increasing?
$(-\infty,1)$
$(0,1)\cup(1,\infty)$
$(0,\infty)$
$(1,\infty)$
$(0,1)$
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.
Let $f'(x)=(x+5)(x-1)^2$. On which interval(s) is $f$ decreasing?
$(1,\infty)$
$(-5,\infty)$
$(-\infty,-5)$
$(-\infty,1)$
$(-5,1)$
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.
Given $f'(x)=-(x-3)^2(x+2)$, on which interval is $f$ decreasing?
$(-\infty,-2)$
$(-2,\infty)$
$(3,\infty)$
$(-\infty,3)$
$(-\infty,-2)$ 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.
Given $f'(x)=\sqrt{x}-2$ with domain $x\ge0$, on which interval(s) is $f$ decreasing?
$(0,4)$
$[0,4)$
$(0,4]$
$(4,\infty)$
$[0,\infty)$
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.
Let $f'(x)=x(x-6)(x+2)$. On which interval(s) is $f$ increasing?
$(-2,0)\cup(6,\infty)$
$(-\infty,-2)\cup(0,6)$
$(-\infty,0)\cup(6,\infty)$
$(0,\infty)$
$(-2,6)$
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.
Given $f'(x)=\dfrac{(x-1)(x-3)}{x^2}$, on which interval is $f$ increasing?
$(-\infty,0)$ and $(0,1)$
$(-\infty,0)$ and $(0,1)$ and $(3,\infty)$
$(0,1)$ and $(3,\infty)$
$(1,3)$ only
$(-\infty,0)$ and $(1,3)$
Explanation
This question tests the skill of determining intervals where a function is increasing by analyzing the sign of its first derivative. The derivative f'(x) = $(x-1)(x-3)/x^2$ indicates that the function f(x) is increasing where f'(x) > 0 and decreasing where f'(x) < 0, with a vertical asymptote at x = 0. The critical points are x = 1 and x = 3, and the denominator is positive for x ≠ 0, so the sign follows the numerator, dividing intervals into (-∞, 0), (0, 1), (1, 3), and (3, ∞). Testing shows f'(x) > 0 in (-∞, 0), (0, 1), and (3, ∞), while negative in (1, 3), meaning f is increasing there. A tempting distractor is choice A, (0, 1) and (3, ∞), but it omits (-∞, 0) where f'(x) > 0 as well. 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.
For $f'(x)=\dfrac{x-5}{(x+1)^2}$, on which interval(s) is $f$ decreasing?
$(-\infty,-1)$
$(-\infty,-1)\cup(-1,5)$
$(-\infty,5)$
$(5,\infty)$
$(-1,5)$
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-5)/(x+1)², critical points are x=5 (numerator zero) and x=-1 (denominator zero, undefined). The sign of f' depends on (x-5) over positive (x+1)². f'<0 when x-5<0, i.e., x<5, but excluding x=-1 where undefined. So decreasing on (-∞,-1) and (-1,5). A tempting distractor like B, (-1,5), fails because it omits the left interval where f' is also negative. Always create a sign chart for the derivative to systematically determine intervals of increase and decrease.
For $f(x)=x^3-6x^2+9x+2$, on which intervals is $f$ increasing?
$(3,\infty)$
$(-\infty,3)$
$(-\infty,1)\cup(3,\infty)$
$(-\infty,1)$
$(1,3)$
Explanation
This question tests your ability to determine where a function is increasing by analyzing the sign of its derivative. To find where f(x) = x³ - 6x² + 9x + 2 is increasing, we first compute f'(x) = 3x² - 12x + 9 = 3(x² - 4x + 3) = 3(x - 1)(x - 3). The derivative equals zero when x = 1 and x = 3, dividing the number line into three intervals: (-∞, 1), (1, 3), and (3, ∞). Testing values in each interval: for x = 0, f'(0) = 3(-1)(-3) = 9 > 0; for x = 2, f'(2) = 3(1)(-1) = -3 < 0; for x = 4, f'(4) = 3(3)(1) = 9 > 0. Choice B might tempt students who confuse increasing with decreasing intervals. The key strategy is to factor the derivative completely, find critical points, then test the sign in each resulting interval.
Given $f'(x)=\dfrac{(x-3)(x+1)}{(x-3)^2+1}$, on which interval(s) is $f$ increasing?
$(-\infty,-1)$
$(-\infty,-1)\cup(3,\infty)$
$(3,\infty)$
$(-1,\infty)$
$(-1,3)$
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) = (x - 3)(x + 1)/((x - 3)² + 1) > 0. Denominator always positive, numerator positive on (-∞, -1) ∪ (3, ∞). Thus, f increases there. A tempting distractor is choice B (-1, 3), but there f' < 0, so f decreases. 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.
Given $f(x)=\dfrac{x^2-1}{x}$ with $x\ne0$, on which interval(s) is $f$ increasing?
$(-1,1)$
$(-1,0)\cup(0,1)$
$(-\infty,-1)\cup(0,1)$
$(-\infty,-1)\cup(1,\infty)$
$(-\infty,0)\cup(0,\infty)$
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. For f(x) = (x² - 1)/x (x ≠ 0), f'(x) = 1 + 1/x² > 0 always where defined. This is positive on (-∞, 0) ∪ (0, ∞). No regions where negative. A tempting distractor is choice E (-1, 1), but f' > 0 there too, though it's not the full set. 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.