Mathematics

Mathematics

Problem

On the curve x2=y3−3yx^{2}=y^{3}-3y, identify all critical points. For any critical point where d2ydx2=0\dfrac{d^{2}y}{dx^{2}}=0, explain briefly why the second–derivative test is inconclusive there and state whether the point is a local extremum or a saddle point.
Intuitive explanation

This problem asks you to find where the curve has horizontal or vertical tangents (critical points), then use the second derivative to classify them. The key insight is that you'll need implicit differentiation to find dydx\frac{dy}{dx} and d2ydx2\frac{d^2y}{dx^2}, and critical points occur where dydx=0\frac{dy}{dx} = 0 or is undefined. When d2ydx2=0\frac{d^2y}{dx^2} = 0 at a critical point, the second-derivative test fails because you can't determine concavity—you'll need to analyze the curve's behavior directly near that point to decide if it's a local extremum or saddle point.

Intuitive explanation

This problem asks you to find where the curve has horizontal or vertical tangents (critical points), then use the second derivative to classify them. The key insight is that you'll need implicit differentiation to find dydx\frac{dy}{dx} and d2ydx2\frac{d^2y}{dx^2}, and critical points occur where dydx=0\frac{dy}{dx} = 0 or is undefined. When d2ydx2=0\frac{d^2y}{dx^2} = 0 at a critical point, the second-derivative test fails because you can't determine concavity—you'll need to analyze the curve's behavior directly near that point to decide if it's a local extremum or saddle point.

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