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...写错了，是3\4sin^2a+1\2+sin2a