该三角函数如何得出来的?(sinx)2(平方)=(1-cos2x)/2
由倍角公式cos2x=2(cosx)^2-1
=2[1-(sinx)^2]-1
=1-2（sinx）^2
所以1-cos2x=2（sinx）^2
（sinx）^2=(1-cos2x)/2
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