韦达定理：
tana+tanb=-4,tanatanb=3
tan(a+b)=(tana+tanb)/(1-tanatanb)=2
cos^2(a+b)=1/5
3cos(a+b)^2+sin(a+b)cos(a+b)
=cos(a+b)^2[3+tan(a+b)]
=1/5*(3+2)
=1