Question

cos^4(x)-sin^4(x)等于?是cos^2(x)还是cos2x

Answer 1

cos^4(x)-sin^4(x)

=[cos^2(x)-sin^2(x)][cos^2(x)+sin^2(x)] (其中[cos^2(x)+sin^2(x)]=1)

=[cos^2(x)-sin^2(x)] （就是cos2x的展开式 ）

=cos2x