解：tana=3--->sina/cosa=3--->sina=3cosa. 1)原式=(4*3cosa-2cosa)/(5cosa-3*3cosa) =10coda/(-4cosa) =10/(-4)=-5/2. 2)sinacosa=3(cosa)^2 =3*1/(seca)^2 =3/(1+(tana)^2] =3/(1+3^2) =1/10. 3)(sina+cosa)^2 =(sina+cosa)^2/[(sina)^2+(cosa)^2]分子、分母同时除以(cosa)^2得到 =(tana+1)^2/[(tana)^2+1] =(3+1)^2/(3^2+1) =16/10=8/5.
答：a,b∈R+ ==> a+b≥2*根号(ab) ==> 1/ab ≥4 a+b=1: (1/a^2 -1)(1/b^2 -1) =[1 -(a^2 +b^2)]...详情>>
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