tana=3
2/3sin^2a+1/4cos^2
=[2/3sin^2a+1/4cos^2]/(sin²a+cos²a)
分子分母同除以cos²a
=[4/3tana+1/4]/[tan²a+1]
=(4+1/4)/10
=17/4*1/10
=17/40为什么2/3sin^2a+1/4cos^2除以cos²a=4/3tana+1/4不是=2/3tana+1/4?sin2a=2sinacosa所以 2/3sin^2a+1/4cos^2除以cos²a= 4/3sinacosa+1/4cos^2除以cos²a=4/3tana+1/4