(1）sina=3cosa; 所以sin²a=9cos²a; 又sina²+cos²a=1
故10cos²a=1; cos²a=1/10;
sin²a+3sinacosa=9cos²a+9cos²a=18/10=9/5;
(2)sina=3cosa; 则tana=3;
sin²a+3sinacosa=(sin²a+3sinacosa)/(sin²a+cos²a)
=(tan²a+3tana)/(tan²a+1)=(9+9)/(9+1)=9/5(sin²a+3sinacosa)/(sin²a+cos²a)为什么会等于(tan²a+3tana)/(tan²a+1)???分子分母同除以cos²a,弦变切