I need to find $\frac{\partial^2x}{\partial t^2}$, where $x = r\sin t$ and $y = r\cos t$.
I get the following:
$\frac{\partial^2z}{\partial x^2} r^2\sin^2 t + 2\frac{\partial^2z}{\partial y\partial x} r^2\cos t\sin t + \frac{\partial^2z}{\partial y^2} r^2\cos^2t$
I heard that it's something else with 6 terms instead of 4, how come?
I don't understand. Please explain it to me in a way my feeble mind can understand.