Question

高二推理证明 急1-tana/2+tana=1 求证3sin2a=-4cos2a我已经得出tana=-1/2sina/c

Answer 1

(sina)^2+(cosa)^2=1

所以(sina)^2+4(sina)^2=1

(sina)^2=1/5

3sin2a=6sinacosa=-12(sina)^2=-12/5

-4cos2a=-4[1-2(sina)^2]=-4+8(sina)^2=-12/5

命题得证