试证
=(sin(c-a)sin(a-b))/sin(b+c) · [-2 sin(b+c) sin(b-c) ]
=-2sin(b-c)sin(c-a)sin(a-b)
试证
=(sin(c-a)sin(a-b))/sin(b+c) · [-2 sin(b+c) sin(b-c) ]
=-2sin(b-c)sin(c-a)sin(a-b)
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