已知sin(a+b)=-3/5,sin(a-b)=3/5,且a-b∈(∏/2,∏),a+b∈(3∏/2,2∏),求cos2b
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![已知sin(a+b)=-3/5,sin(a-b)=3/5,且a-b∈(∏/2,∏),a+b∈(3∏/2,2∏),求cos2b](/uploads/image/z/14570405-53-5.jpg?t=%E5%B7%B2%E7%9F%A5sin%28a%2Bb%29%3D-3%2F5%2Csin%28a-b%29%3D3%2F5%2C%E4%B8%94a-b%E2%88%88%28%E2%88%8F%2F2%2C%E2%88%8F%29%2Ca%2Bb%E2%88%88%EF%BC%883%E2%88%8F%2F2%2C2%E2%88%8F%EF%BC%89%2C%E6%B1%82cos2b)
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已知sin(a+b)=-3/5,sin(a-b)=3/5,且a-b∈(∏/2,∏),a+b∈(3∏/2,2∏),求cos2b
已知sin(a+b)=-3/5,sin(a-b)=3/5,且a-b∈(∏/2,∏),a+b∈(3∏/2,2∏),求cos2b
已知sin(a+b)=-3/5,sin(a-b)=3/5,且a-b∈(∏/2,∏),a+b∈(3∏/2,2∏),求cos2b
a-b∈(∏/2,∏),a+b∈(3∏/2,2∏)
所以cos(a+b)=4/5 cos(a-b)=-4/5
cos2b=cos[(a+b)-(a-b)]=cos(a+b)cos(a-b)+sin(a+b)sin(a-b)
=4/5*(-4/5)+(-3/5)*3/5
=-1
7/25
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