sin(x+y)*sin(y-x)=m,cosx的平方-cosy的平方?
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sin(x+y)*sin(y-x)=m,cosx的平方-cosy的平方?
原式=(cos x + cos y )*(cos x - cos y )
={cos[(x+y)/2+(x-y)/2]+cos[(x+y)/2-(x-y)/2]}
*{cos[(x+y)/2+(x-y)/2]-cos[(x+y)/2-(x-y)/2]}
=-4cos[(x+y)/2]*cos[(x-y)/2]*sin[(x+y)/2]*sin[(x-y)/2]
=-{2*sin[(x+y)/2]*cos[(x+y)/2]}{2*sin[(x-y)/2]*cos[(x-y)/2]}
=-sin(x+y)*sin(x-y)
=sin(x+y)*sin(y-x)
=m
={cos[(x+y)/2+(x-y)/2]+cos[(x+y)/2-(x-y)/2]}
*{cos[(x+y)/2+(x-y)/2]-cos[(x+y)/2-(x-y)/2]}
=-4cos[(x+y)/2]*cos[(x-y)/2]*sin[(x+y)/2]*sin[(x-y)/2]
=-{2*sin[(x+y)/2]*cos[(x+y)/2]}{2*sin[(x-y)/2]*cos[(x-y)/2]}
=-sin(x+y)*sin(x-y)
=sin(x+y)*sin(y-x)
=m
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