用a代替cosa=7/5-sina平方cos²a=1-sin²a=49/25-14/5*sina+sin²asin²a-7/5*sina+12/25=0(sina-4/5)(sina-3/5)=0sina=4/5,sina=3/5所以sina=4/5,cosa=3/5sina=3/5,cosa=4/5