Question

(cosa)^2(cosb)^2-(sina)^2(sinb)^2.为什么等于.(cosa)^2-(sinb)^2

Answer 1

(cosa)^2(cosb)^2-(sina)^2(sinb)^2

=(cosa)^2[1-(sinb)^2]-[1-(cosa)^2](sinb)^2

=(cosa)^2-(cosa)^2(sinb)^2-(sinb)^2+(cosa)^2(sinb)^2

=(cosa)^2-(sinb)^2