cos^4( pai/8)-sin^4( pai/8)
问题描述：
cos^4( pai/8)-sin^4( pai/8)
答
cos^4( pai/8)-sin^4( pai/8)
=(cos^2( pai/8)-sin^2( pai/8))(cos^2( pai/8)+sin^2( pai/8))
=cos^2( pai/8)-sin^2( pai/8)
=cos(pai/4)
=√2/2