sinx=2sin(x/2)*cos(x/2)
cosx+1=[cos(x/2)]^2-[sin(x/2)]^2
+[cos(x/2)]^2-[sin(x/2)]^2
=2[cos(x/2)]^2
∴sinx/(1+cosx)= 2sin(x/2)*cos(x/2)/2[cos(x/2)]^2=tanx/2
得证!
tanx/2=sinx/1+cosx求证
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