sin(a+π）=4/5-sina=4/5sina=-4/5sinacosa＜0那么cosa>0sin²a+cos²a=1求得：cosa=3/5tana=-4/32sin（a-π）+3tan(3π-a)/4cos(a-3π）=-2sina+3tana/4cosa=8/5+(-4)/(12/15)=8/5-5=-17/5