In ΔABC,(a−b)2cos2C2+(a+b)2sin2C2 is equal to
a2
b2
c2
2a2
(a−b)2cos2C2+(a+b)2sin2C2=(a2+b2−2ab)cos2C2+(a2+b2+2ab)sin2C2=(a2+b2)+2ab(sin2C2−cos2C2)a2+b2−2ab cosC=a2+b2−(a2+b2−c2)=c2