Question
If $${x^2} + {y^2} = 1,$$ then
A.
$$yy'' - 2{\left( {y'} \right)^2} + 1 = 0$$
B.
$$yy'' + {\left( {y'} \right)^2} + 1 = 0$$
C.
$$yy'' + {\left( {y'} \right)^2} - 1 = 0$$
D.
$$yy'' + 2{\left( {y'} \right)^2} + 1 = 0$$
Answer :
$$yy'' + {\left( {y'} \right)^2} + 1 = 0$$
Solution :
Given $${x^2} + {y^2} = 1.$$
Differentiating w.r.t. $$x,$$ we get $$2x + 2yy' = 0\,\,{\text{or }}x + yy' = 0$$
Again differentiating w.r.t. $$x,$$ we get $$1 + y'y' + yy'' = 0\,\,{\text{or 1}} + {\left( {y'} \right)^2} + yy'' = 0$$