cos(a+b)=12/13,cos(a-b)=-12/13且a-b属于(pi/2,pi),a+b属于(3pi/2,2pi)求cos2a-cos2b
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已知cos(a-b)=-12/13 cos(a+b)=12/13且π/2
cos(a+b)=12/13,cos(2a+b)=3/5,求cosa
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