A fellow student asks you, \In elementary calculus we found minima of a function by differentiating it, setting the resulting expression to zero, and then solving the equation. Since our loss function...

A fellow student asks you, \In elementary calculus we found minima of a function by differentiating it, setting the resulting expression to zero, and then solving the equation. Since our loss function is differentiable, why don't we do that rather than bothering with gradient descent?" Explain why this is not, in fact, possible.