Question

Use the chain rule to find dw/dt. w = ln x2 + y2 + z2 , x = 7 sin(t), y = 9 cos(t), z = 6 tan(t)

Answer 1

By the chain rule,

$\dfrac{\mathrm dw}{\mathrm dt}=\dfrac{\partial w}{\partial x}\dfrac{\mathrm dx}{\mathrm dt}+\dfrac{\partial w}{\partial y}\dfrac{\mathrm dy}{\mathrm dt}+\dfrac{\partial w}{\partial z}\dfrac{\mathrm dz}{\mathrm dt}$

We have

$\dfrac{\partial w}{\partial x}=\dfrac{2x}{x^2+y^2+z^2}$
$\dfrac{\partial w}{\partial y}=\dfrac{2y}{x^2+y^2+z^2}$
$\dfrac{\partial w}{\partial z}=\dfrac{2z}{x^2+y^2+z^2}$

$\dfrac{\mathrm dx}{\mathrm dt}=7\cos t$
$\dfrac{\mathrm dy}{\mathrm dt}=-9\sin t$
$\dfrac{\mathrm dz}{\mathrm dt}=6\sec^2t$

So we have

$\dfrac{\mathrm dw}{\mathrm dt}=\dfrac2{x^2+y^2+z^2}\left(7x\cos t-9y\sin t+6z\sec^2t\right)$
$\dfrac{\mathrm dw}{\mathrm dt}=\dfrac{2\left(49\sin t\cos t-81\cos t\sin t+36\tan t\sec^2t\right)}{49\sin^2t+81\cos^2t+36\tan^2t}$
$\dfrac{\mathrm dw}{\mathrm dt}=\dfrac{8\sin t(9-16\cos^4t)}{\cos t(36+13\cos^2t+32\cos^4t)}$