cos(a-π/4）=1/4,则sin2a
问题描述：
cos(a-π/4）=1/4,则sin2a
答
cos(a-π/4）=1/4，倍角公式cos(2a-π/2）=sin2a=2cos(a-π/4）^2-1=2*(1/4)^2-1=-7/8，故sin2a=-7/8
答
=-7/16，你把等式右边展开。再两边平方，答案就显现出来了。
答
cos(a-π/4)=cosacosπ/4+sinasinπ/4=√2/2cosa+√2/2sina=√2/2(cosa+sina)=1/4sina+cosa=√2/4sin2a=2sinacosa=(sina+cosa)²-(sin²a+cos²a)=1/8-1=-7/8
答
sin2a=cos(2a-π/2)=2(cos(a-π/4))^2-1=-7/8