दीर्घवृत्तों \(\mathrm{E}_{\mathrm{k}}: \mathrm{kx}^2+\mathrm{k}^2 \mathrm{y}^2=1, \mathrm{k}=1,2, \ldots ., 20\) का विचार कीजिए। माना \(C_k\) वह वृत्त है, जो दीर्घवृत्त \(E_k\) के अन्त्य बिंदुओं (एक लघु अक्ष पर तथा दूसरा दीर्घ अक्ष पर) को मिलाने वाली चार जीवाओं को स्पर्श करता है। यदि वृत्त \(C_k\) की त्रिज्या \(r_k\) है, तो \(\sum_{\mathrm{k}=1}^{20} \frac{1}{\mathrm{r}_{\mathrm{k}}^2}\) का मान है :

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.