Can u explain how they proceed in this sum??

Exampi Let D {(r, y):x20,y20). Letf(a,y) = (r* + J) ** for (,)ED. Prove thatfattains its maximum on D at two boundary points. Deduce that -se"** for all x2 0,y2 0. Solution Sx )= +¥) ** Converting it into polar co-ordinate x= rcos 0,y=rsin0 So, Sr,0)=r'e"lcon0osin@ Since, x20,y20, thus 0 will vary from 0 to Maximum off(r,0 )will occur where sin 0 + cos0 is minimum. In the interval minimum value of sin 6 + cos 0 will be l ie. in the direction of either 6-0 or - Thus, maximum will occur in the dire