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Question

Find all points (x, y) at which the tangent plane to the graph of z=x3z=x^3 is horizontal.

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Answered 2 years ago
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z=f(x,y)=x3z=f\left( x,y\right)=x^{3}

f(x,y)=f(x,y)xi^+f(x,y)xj^=3x2i^+0j^=3x2\begin{align*} \nabla f\left(x,y \right)&=\dfrac{\partial f\left(x,y \right)}{\partial x}\hat{i} +\dfrac{\partial f\left(x,y \right)}{\partial x}\hat{j}\\ &=3x^{2}\hat{i} +0\hat{j} \\ &=3x^{2}\end{align*}

Tangent plane is horizontal then zz constant

f(x,y)=0,03x2,0=0,0x=0\begin{align*} \nabla f\left( x,y\right)&=\left\langle 0,0\right\rangle\\ \left\langle 3x^{2},0 \right\rangle&=\left\langle 0,0\right\rangle\\ x&=0\end{align*}

we conclude ,the tangent plane is horizontal at point (0,0)\left(0,0\right)

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