If the normal at the point $t_1$ (i.e., at $\left.\left(a t_1^2, 2 \mathrm{at}_1\right)\right)$ on $y^2=4 a…
If the normal at the point $t_1$ (i.e., at $\left.\left(a t_1^2, 2 \mathrm{at}_1\right)\right)$ on $y^2=4 a x$ meets the parabola again at the point $t_2$, then $t_1 t_2=$