sina-2cosa=0
sina=2cosa
sina-3cosa/sina+3cosa
=2cosa-3cosa / 2cosa+3cosa
=-cosa/5cosa
=-1/5已知sina-2cosa=0求sin²a＋sinacosa＋3谢谢sin²a＋sinacosa＋3=(sin²a＋sinacosa)/(sin²a+cos²a) +3=(4cos²a+2cos²a)/(5cos²a)=6/5+3=21/5