cos^4(π/8)-sin^4(π/8)等于?
问题描述：
cos^4(π/8)-sin^4(π/8)等于?
答
cos^4(π/8)-sin^4(π/8)[平方差公式]=(cos^2(π/8)-sin^2(π/8))(cos^2(π/8)+sin^2(π/8))=(cos^2(π/8)-sin^2(π/8))*1[2倍角公式 cos^2(a)-sin^2(a)=cos2a]=cos(π/8*2)=cos(π/4)=根号2/2