已知tan(π-a)=2,求值:(1)2sin+2cosa/7sina+cosa(2)sina*cosa(3)4cosa^2+3sina^2
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![已知tan(π-a)=2,求值:(1)2sin+2cosa/7sina+cosa(2)sina*cosa(3)4cosa^2+3sina^2](/uploads/image/z/2667528-0-8.jpg?t=%E5%B7%B2%E7%9F%A5tan%28%CF%80-a%29%3D2%2C%E6%B1%82%E5%80%BC%EF%BC%9A%EF%BC%881%EF%BC%892sin%2B2cosa%2F7sina%2Bcosa%EF%BC%882%EF%BC%89sina%2Acosa%EF%BC%883%EF%BC%894cosa%5E2%2B3sina%5E2)
已知tan(π-a)=2,求值:(1)2sin+2cosa/7sina+cosa(2)sina*cosa(3)4cosa^2+3sina^2
已知tan(π-a)=2,求值:(1)2sin+2cosa/7sina+cosa(2)sina*cosa(3)4cosa^2+3sina^2
已知tan(π-a)=2,求值:(1)2sin+2cosa/7sina+cosa(2)sina*cosa(3)4cosa^2+3sina^2
tan(π-a)=2
-tana=2
tana=-2
(2sin+2cosa)/(7sina+cosa)
=2(sina+cosa)/(7sina+cosa)分子分母同时除以cosa
=2(sina/cosa+cosa/cosa)/(7sina/cosa+cosa/cosa)
=2(tana+1)/(7tana+1)
=2*(-2+1)/(-2*7+1)
=-2/(-13)
=2/13
sina*cosa
=sina*cosa/1
=sina*cosa/(sin²a+cos²a)分子分母同时除以cos²a
=(sina*cosa/cos²a)/(sin²a/cos²a+cos²a/cos²a)
=tana/(tan²a+1)
=-2/[(-2)²+1]
=-2/5
4cosa^2+3sina^2
=(4cosa^2+3sina^2)/1
=(4cosa^2+3sina^2)/(sin²a+cos²a)分子分母同时除以cos²a
=(4cosa^2/cos²a+3sina^2/cos²a)/(sin²a/cos²a+cos²a/cos²a)
=(4+3tan²a)/(tan²a+1)
=[4+3*(-2)²]/[(-2)²+1]
=16/5