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CBSE Class 10 & 12 boards2026–27 Academic Session
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A good method.
One sign to fix.

Read Vedora’s full grading
You used parametric differentiation which is the right idea. You made a sign error when differentiating x = a cos³t and that led to a wrong final choice. Fix the sign and the answer becomes D (1).
The question
For x = a cos³t and y = a sin³t, find dy/dx at t = 3π/4.
(A) −1 · (B) −1/√3 · (C) −√3 · (D) 1
What went well
- Used dy/dx = (dy/dt)/(dx/dt).
- Found dy/dt = 3a sin²t cos t correctly.
- Evaluated tan(3π/4) correctly, although it followed an incorrect derivative.
Why Vedora awarded 1/5
Partial credit was given for the correct dy/dt. The missing negative sign in dx/dt affected the ratio and the final answer.
What to improve · Moderate
When differentiating cos³t, include the inner derivative: d(cos t)/dt = −sin t. Practise powers of sine and cosine and check the sign at each step.
The complete result
dx/dt = −3a cos²t sin t
dy/dt = 3a sin²t cos t
dy/dx = −tan t
At t = 3π/4: −tan(3π/4) = +1
The correct choice is D (1).
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