Sum of inverse squares of roots

If $\alpha$, $\beta$ are the roots of the equation $a{x}^2+bx+c=0$, find the value of $\frac{1}{{\alpha }^2}+\frac{1}{{\beta }^2}$.

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. IX. The Theory of Quadratic Equations, Examples IX.a, No. 18