计算sin^4(3π/8)-cos^4(3π/8)
问题描述：
计算sin^4(3π/8)-cos^4(3π/8)
答
原式=[sin^2(3π/8)-cos^2(3π/8) ][sin^2(3π/8)+cos^2(3π/8) ] =[sin^2(3π/8)-cos^2(3π/8)] =[sin^2(3π/8)-(1-sin^2(3π/8)} =2sin^2(3π/8)-1 利用二倍角公式可得 =1-cos3π/4-1 =-cos3π/4 =-cos（π-π/4）...