y2=16x;a=4 focus S≡(4,0)

2at1=−4⇒2(4)t1=−4⇒t1=−21∵t1t2=−1
$\begin{aligned}
& \Rightarrow \mathrm{t}_2=2 \
& \therefore \mathrm{Q}\left(\mathrm{at}_2^2, 2 \mathrm{at}_2\right)=(16,16)
\end{aligned}$
Let, S divides PQ internally in λ:1 ratio
$\begin{aligned}
& \therefore \frac{16 \lambda-4}{\lambda+1}=0 \
& \lambda=\frac{1}{4}=\frac{\mathrm{m}}{\mathrm{n}} \
& \therefore \mathrm{~m}^2+\mathrm{n}^2=1+16=17
\end{aligned}$