Mathematics

Mathematics

Problem

For the rose curve r=2sin⁡2θr=2\sin 2\theta, how many points of inflection occur in the interval 0<θ<π20<\theta<\dfrac{\pi}{2}? a) Exactly one b) Exactly two c) None d) Exactly four
Intuitive explanation

A rose curve traces out petals as θ\theta varies, and an inflection point is where the curve's concavity switches. The key is to find where the second derivative d2ydx2\frac{d^2y}{dx^2} changes sign—not just where it equals zero. You'll need to convert to Cartesian coordinates using x=rcos⁡θx = r\cos\theta and y=rsin⁡θy = r\sin\theta, then compute d2ydx2\frac{d^2y}{dx^2} using the chain rule carefully. The inflection points won't necessarily align with where rr or drdθ\frac{dr}{d\theta} equal zero; you're looking for genuine changes in the curve's bending direction.

Intuitive explanation

A rose curve traces out petals as θ\theta varies, and an inflection point is where the curve's concavity switches. The key is to find where the second derivative d2ydx2\frac{d^2y}{dx^2} changes sign—not just where it equals zero. You'll need to convert to Cartesian coordinates using x=rcos⁡θx = r\cos\theta and y=rsin⁡θy = r\sin\theta, then compute d2ydx2\frac{d^2y}{dx^2} using the chain rule carefully. The inflection points won't necessarily align with where rr or drdθ\frac{dr}{d\theta} equal zero; you're looking for genuine changes in the curve's bending direction.

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