cos^4(x)-sin^4(x)+2sin^x的值等于
问题描述：
cos^4(x)-sin^4(x)+2sin^x的值等于
答
cos^4(x)-sin^4(x)+2sin^2(x)
=[cos^2(x)+sin^2(x)][cos^2(x)-sin^2(x)]+2sin^2(x) 【cos^2(x)+sin^2(x)=1】
=cos^2(x)-sin^2(x)+2sin^2(x)
=cos^2(x)+sin^2(x)
=1