Differentiating Inverse Trigonometric Functions

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AP Calculus AB › Differentiating Inverse Trigonometric Functions

Questions 1 - 10
1

A model defines $k(x)=\arcsec(2-x)$. What is $k'(x)$?

$-\sec^2(2-x)$

$\dfrac{1}{|2-x|\sqrt{(2-x)^2-1}}$

$-\dfrac{1}{(2-x)\sqrt{(2-x)^2-1}}$

$-\dfrac{1}{|2-x|\sqrt{(2-x)^2-1}}$

$-\dfrac{1}{\sqrt{(2-x)^2-1}}$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arcsec function. Derivative of arcsec(u) with u as 2-x is 1 over |u| sqrt(u squared minus 1) times -1. Gives -1 over |2-x| sqrt(...). Negative from decreasing u. Choice A uses positive, ignoring u' sign. Pattern: Decreasing arguments in arcsec introduce a negative sign via chain rule.

2

A signal is modeled by $y(x)=\arcsin(2x)$. What is $y'(x)$?

$\dfrac{2}{\sqrt{1+4x^2}}$

$\dfrac{2}{\sqrt{1-4x^2}}$

$\dfrac{-2}{\sqrt{1-4x^2}}$

$\dfrac{1}{\sqrt{1-4x^2}}$

$2\cos(2x)$

Explanation

This problem involves differentiating the inverse sine function with a composite argument. The derivative of arcsin(u) is 1/√(1-u²) times the derivative of u. Since u = 2x, we multiply by 2 (the derivative of 2x). This yields 2/√(1-4x²), where we substitute (2x)² = 4x² in the denominator. A tempting mistake would be to write 1/√(1-4x²), forgetting to apply the chain rule and multiply by 2. When differentiating inverse trig functions, always check if the argument is composite and apply the chain rule accordingly.

3

A control angle is given by $f(x)=\arctan(1-4x)$. What is $f'(x)$?

$-4\sec^2(1-4x)$

$\dfrac{-4}{1+(1-4x)^2}$

$\dfrac{-4}{\sqrt{1+(1-4x)^2}}$

$\dfrac{-1}{1+(1-4x)^2}$

$\dfrac{4}{1+(1-4x)^2}$

Explanation

This problem involves differentiating arctangent of a linear expression. The derivative of arctan(u) is 1/(1+u²) times the derivative of u. Here, u = 1-4x, so we multiply by -4 (the derivative of 1-4x). This yields -4/(1+(1-4x)²), keeping the entire expression (1-4x) squared in the denominator. A student might incorrectly write 4/(1+(1-4x)²) by missing the negative sign from the derivative of 1-4x. When the inner function decreases (has a negative derivative), the overall derivative must reflect this negative contribution.

4

In navigation, $p(u)=\arccos(1-u)$ represents a heading correction. What is $p'(u)$?

$\dfrac{1}{\sqrt{1-(1-u)^2}}$

$-\dfrac{1}{\sqrt{1+(1-u)^2}}$

$-\sin(1-u)$

$-\dfrac{1}{\sqrt{1-(1-u)^2}}$

$\dfrac{1}{1+(1-u)^2}$

Explanation

This problem involves differentiating arccos with argument 1-u. The derivative of arccos(u) is -1/√(1-u²), and with u = 1-u (note the variable), we apply the chain rule. Computing: p'(u) = -1/√(1-(1-u)²) · d/du(1-u) = -1/√(1-(1-u)²) · (-1) = 1/√(1-(1-u)²). Option B incorrectly keeps the negative sign, not recognizing that two negatives make a positive. When arccos (negative derivative) contains a decreasing function (negative derivative), the result is positive.

5

For $y=\arcsin(\tfrac{3x+2}{5})$, what is $\dfrac{dy}{dx}$?

$\dfrac{1}{5\sqrt{1-\left(\tfrac{3x+2}{5}\right)^2}}$

$\dfrac{3}{\sqrt{1-\left(\tfrac{3x+2}{5}\right)^2}}$

$\dfrac{3}{5\sqrt{1-\left(\tfrac{3x+2}{5}\right)^2}}$

$\dfrac{3}{5\sqrt{1-\tfrac{3x+2}{5}}}$

$\cos!\left(\tfrac{3x+2}{5}\right)$

Explanation

This question tests the skill of differentiating inverse trigonometric functions, specifically the inverse sine. The derivative of arcsin(u) is 1/√(1 - u²) times du/dx. u = (3x + 2)/5, du/dx = 3/5, positive. Uses sine identity. Result: (3/5) / √(1 - ((3x + 2)/5)²). Choice A omits 5, wrong scaling. Pattern: arcsin positive with positive u'.

6

A rate model uses $s(x)=\arccos(7-2x)$. What is $s'(x)$?

$\dfrac{2}{\sqrt{1-(7-2x)}}$

$-2\sin(7-2x)$

$-\dfrac{2}{\sqrt{1-(7-2x)^2}}$

$\dfrac{2}{\sqrt{1-(7-2x)^2}}$

$\dfrac{1}{\sqrt{1-(7-2x)^2}}$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arccos function. The derivative of arccos(u) with u as 7-2x is -1 over sqrt(1 minus u squared) times u', where u' is -2. Multiplying yields positive 2 over the sqrt term due to the negatives canceling. This illustrates how the chain rule can flip signs based on the inner derivative. Choice B forgets to account for the positive outcome from the chain rule. Recognize patterns in arccos with decreasing inners by checking if signs cancel to positive.

7

In a dynamics model, $t(x)=\arctan(1-3x)$. What is $t'(x)$?

$\dfrac{3}{1+(1-3x)^2}$

$\dfrac{-3}{1+(1-3x)^2}$

$\dfrac{-3}{1+(1-3x)}$

$-3\sec^2(1-3x)$

$\dfrac{1}{1+(1-3x)^2}$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arctan function. For arctan(u) with u as 1-3x, the derivative is 1 over 1 plus u squared times u', which is -3. This results in -3 over 1 plus (1-3x) squared, incorporating the chain rule. The negative reflects the decreasing inner function. Choice A ignores the negative from u', leading to a sign error. Spot arctan patterns with negative coefficients by ensuring the multiplier carries the sign.

8

A control system uses $h(x)=\arctan(5x+4)$. What is $h'(x)$?

$\dfrac{5}{1+(5x+4)^2}$

$\dfrac{1}{1+(5x+4)^2}$

$-\dfrac{5}{1+(5x+4)^2}$

$\dfrac{5}{1+(5x+4)}$

$5\sec^2(5x+4)$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arctan function. The formula for the derivative of arctan(u) is 1 over 1 plus u squared, multiplied by u'. Here, u is 5x+4, so u' is 5, yielding 5 over 1 plus (5x+4) squared after chain rule application. This shows how the derivative measures sensitivity to changes in the linear argument. Choice A errs by placing the 5 in the numerator without squaring the denominator term correctly. Recognize arctan patterns by ensuring the chain rule multiplier matches the coefficient of x in the argument.

9

A response curve uses $w(x)=\arcsin(\tfrac{x}{4})$. What is $w'(x)$?

$\dfrac{1}{4\sqrt{1-(x/4)^2}}$

$\dfrac{1}{\sqrt{1-(x/4)^2}}$

$\dfrac{1}{\sqrt{1-x/4}}$

$\dfrac{1}{4\sqrt{1-x^2}}$

$\cos(\tfrac{x}{4})$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arcsin function. Derivative of arcsin(u) with u as x/4 is 1 over sqrt(1 minus u squared) times 1/4. This yields 1/4 over sqrt(1 minus (x/4) squared), adjusting for the scaled input. It shows how fractions in arguments reduce the derivative's magnitude. Choice A forgets the 1/4 chain rule factor, overestimating the rate. Recognize scaled arcsin by dividing the standard derivative by the denominator in u.

10

A sensor model uses $f(x)=\arcsin(3x)$. What is $f'(x)$?

$3\cos(3x)$

$\dfrac{3}{\sqrt{1-9x^2}}$

$-\dfrac{3}{\sqrt{1-9x^2}}$

$\dfrac{1}{\sqrt{1-9x^2}}$

$\dfrac{3}{\sqrt{1-3x}}$

Explanation

This problem tests the skill of differentiating inverse trigonometric functions, specifically the arcsin function. The derivative of arcsin(u) is 1 over the square root of 1 minus u squared, multiplied by the derivative of u. Here, u is 3x, so u' is 3, and we multiply the standard derivative by 3 to account for the chain rule. This results in 3 divided by the square root of 1 minus 9x squared, capturing how the rate of change scales with the coefficient. A tempting distractor is choice B, which omits the chain rule multiplier of 3 and underestimates the derivative. To recognize similar patterns, look for arcsin of a linear term and always multiply by the inner function's derivative.

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