2(sin^2)a-sinacosa+(cos^2)a=【2(sin^2)a-sinacosa+(cos^2)a】/【(sin^2)a+(cos^2)a】（分子、分母同时除以(cos^2)a,得=【2tan^2a-tana+1】/（tan^2+1）因为tana=2,所以原式=（2*2^2-2+1)/(2^2+1)=9/5....