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Derivatives & Second-Order Derivatives · Question 50 of 96

Question 50

MediumOriginal question

If x=cos⁡3tx = \cos^3 t and y=sin⁡3ty = \sin^3 t, what is dydx\frac{dy}{dx}?

Options for question 50

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. dxdt=−3cos⁡2tsin⁡t\frac{dx}{dt} = -3\cos^2 t\sin t and dydt=3sin⁡2tcos⁡t\frac{dy}{dt} = 3\sin^2 t\cos t.

  2. dydx=3sin⁡2tcos⁡t−3cos⁡2tsin⁡t=−sin⁡tcos⁡t=−tan⁡t\frac{dy}{dx} = \frac{3\sin^2 t\cos t}{-3\cos^2 t\sin t} = -\frac{\sin t}{\cos t} = -\tan t.

Worth noting

tan⁡t\tan t loses the minus sign that comes from differentiating cos⁡t\cos t.

Concept
Parametric functions with powers of sine and cosine
Typical time
90 s
Your time
—
The rule this testsParametric functions
Practise this chapter →

Ten questions from Derivatives & Second-Order Derivatives, at your own pace.

Practise this chapter →

Ten questions from Derivatives & Second-Order Derivatives, at your own pace.