(3sina+2cosa)^2+(2sina-3cosa)^2=
(3sina+2cosa)^2+(2sina-3cosa)^2=9sin^2a+12sinacosa+4cos^2a +4sin^2a-12sinacosa+
+9cos^2a=13sin^2a+13cos^2a=13(sin^2a+cos^2a)=13