3sinacosa
=3sinacosa/1
=3sinacosa/(cos²a+sin²a)分子分母同时除以cos²a
=(3sinacosa/cos²a)/(cos²a/cos²a+sin²a/cos²a)
=3tana/(1+tan²a)
=3*(-4)/[1+(-4)²]
=-12/17
cos²a-sin²a
=(cos²a-sin²a)/1
=(cos²a-sin²a)/(cos²a+sin²a)分子分母同时除以cos²a
=(cos²a/cos²a-sin²a/cos²a)/(cos²a/cos²a+sin²a/cos²a)
=(1-tan²a)/(1+tan²a)
=[1-(-4)²]/[1+(-4)²]
=-15/17
(4sina-2cosa)/(5cosa+3sina)分子分母同时除以cosa
=(4sina/cosa-2cosa/cosa)/(5cosa/cosa+3sina/cosa)
=(4tana-2)/(5+3tana)
=[4*(-4)-2]/[5+3*(-4)]
=-18/(-7)
=18/7