I don't know how to prove it...

(a) for parametric equations x = f(t) and y = g(t), where t is a parameter, show that

$\displaystyle

\frac{d^2y}{dx^2}\:=

\frac{\frac{d}{dt}(\frac{dy}{dx})} {\frac{dx}{dt}}$

(b) Given the parametric equations

$\displaystyle

x = \frac{t^2}{2}\ and\ y = 1-t $

find$\displaystyle

\frac{d^2y}{dx^2}\$