Let D be the annulus 1

Extracted text: Let D be the annulus 1< 9,="" and="" let="" ca="" denote="" the="" circle="" of="" radius="" a="" centered="" at="" the="" origin="" and="" oriented="" counterclockwise.="" if="" f(r,="" y)="(" in(y+4)="" –="" x²y+xy,="" +="" xy?="" ),="" find="" y+4="" f-="" dr="" f.="" dr.="" select="" one:="" а.="" —="" 12т="" 15л="" b.="" 4="" с.="" 40л="" 117="" d.="" -="" е.="" 24т="">