三角函数证明
sin(a+b)-sin a=2cos(a+b/2)sin(b/2)
sin(a+b)-sina =sinacosb+cosasinb - sina =sina(cosb-1) + cosa[2sin(b/2)cos(b/2)] =sina[-2sin²(b/2)] + cosa[2sin(b/2)cos(b/2)] =2sin(b/2)[cosacos(b/2)-sinasin(b/2)] =2sin(b/2)cos(a+b/2)
sin(a+b)-sin a 直接用和差化积公式 =2cos[(a+b+a)/2]sin[(a+b-b)/2] =2cos(a+b/2)sin(b/2)
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