sin(a-b)是多少？详细推理过程 sin（a+b）的推导过程
sin\uff08a+b\uff09\u7b49\u4e8e\u4ec0\u4e48\uff1f\u5199\u51fa\u8be6\u7ec6\u63a8\u5bfc\u8fc7\u7a0b\u3002sin(a+b)
=cos[\u03c0/2-(a+b)]
=cos[(\u03c0/2-a)-b]
=cos(\u03c0/2-a)cosb+sin(\u03c0/2-a)sinb
=sinacosb+cosasinb
-------------------------------------------------------------
sin(a+b)=cos(\u03c0/2-\uff08a+b))=cos(\uff08\u03c0/2-a\uff09-b\uff09=cos\uff08\u03c0/2-a\uff09cosb+sin\uff08\u03c0/2-a\uff09sinb=sinacosb+cosasinb
sin(a-b)=sinacosb-cosasinb用单位圆证明
x^2+y^2=1
A(x1,y1),B(x2,y2),C(x3,y3)
sinA=y1,cosA=x1,sinB=y2,cosB=x2,
k1=tanA=y1/x1,k2=tanB=y2/x2
k=tan(a+b)=(tana+tanb)/(1-tanatanb)
=(y1x2+y2x1)/(x1x2-y1y2)
y3=y1x2+y2x1,x3=x1x2-y1y2
sin（a+b)=y1x2+y2x1
sinacosb+cosasinb=y1x2+x1y2=y1x2+y2x1=sin(a+b)
b=-b
sin(a-b)=sinacos(-b)+cosasin(-b)=sinacosb-cosasinb
证明完毕。
sin(a-b)=sinacosb-cosasinb
en 内个。。我们学的时候就没有推理。。就是要你死记硬背的公式。。所以。。你现在就只能背下来。。。因为 老师说太复杂所以不说。。。
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