已知sin(π/4-a)=5/13 a∈(0,π/4)求cos2a/cos(π/4+a)
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已知sin(π/4-a)=5/13 a∈(0,π/4)求cos2a/cos(π/4+a)
![已知sin(π/4-a)=5/13 a∈(0,π/4)求cos2a/cos(π/4+a)](/uploads/image/z/1765203-51-3.jpg?t=%E5%B7%B2%E7%9F%A5sin%28%CF%80%2F4-a%EF%BC%89%3D5%2F13+a%E2%88%88%EF%BC%880%2C%CF%80%2F4%EF%BC%89%E6%B1%82cos2a%2Fcos%28%CF%80%2F4%2Ba%29)
sin(派/4-a)=5/13 cos(派/4-a)=12/13 sin(派/2-2a)=2*5/13*12/13=120/169=cos2a cos(派/4+a)=sin(派/2-派/4-a)=sin(派/4-a)=5/13 cos2a/[cos(派/4+a)] =120/169÷5/13 =24/13
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