最后的cos 应该少了1个2次,tanα=sinα/cosα=3,sinα=3cosα,1=sin^2α+cos^2α=9cos^2α+coa^2α=10cos^2α原式=（sin^2α-3sinαcosα+cos^2α)/（sin^2α+cos^2α）=（9cos^2α-9cos^2α+cos^2α)/(10cos^2...