If x = acost, y = asint, then d2ydx2 at t=π4 is
a2√2
−a2√2
2√2a
−2√2a
Clearly x2+y2=a2 and y(π4)=a√2,(π4)=−1 2x+2yy1=0⇒y1=−xy,soy1(π4)=−1.Nowx+yy1=0⇒1+y21+yy2=0 ⇒y2(π4)=−1+(y1(π4))2y(π4)=−2√2a