Question 47If $r$ and $s$ are the roots of the equation $ax^2+bx+c=0,$ the value of $\dfrac1{r^2}+\dfrac1{s^2}$ is:$b^2-4ac$$\dfrac{b^2-4ac}{2a}$$\dfrac{b^2-4ac}{c^2}$$\dfrac{b^2-2ac}{c^2}$None of these