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
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