已知tana=2,求3\4sin^2a+1\2+sin2a写错了，是3\4sin^2a+1\2+sin2a

已知tana=2,求3\4sin^2a+1\2+sin2a写错了，是3\4sin^2a+1\2+sin2a
已知tana=2,求3\4sin^2a+1\2+sin2a
写错了，是3\4sin^2a+1\2+sin2a

已知tana=2,求3\4sin^2a+1\2+sin2a写错了，是3\4sin^2a+1\2+sin2a
3/(4sin^2a+1\2+sin2a)
=3(sin^2a+cos^2a)/(4sin^2a+sin^2a+cos^2a\2+2sinacosa)
=3(sin^2a+cos^2a)/(5sin^2a+cos^2a\2+2sinacosa)分子分母同时除以cos^2a
=3(sin^2a/cos^2a+cos^2a/cos^2a)/(5sin^2a/cos^2a+cos^2a/cos^2a\2+2sinacosa/cos^2a)
=3(tan^2a+1)/(5tan^2a+1\2+2tana)
=3*(2^2+1)/(5*2^2+1\2+2*2)
=15/25
=3/5