b为锐角 sinb=根号10/10 cosb=3根号10/10 tanb=1/3
tan(a+b)=(tana+tanb)/(1-tana*tanb)
=(1/2+1/3)/(1-1/2*1/3)
=(5/6)/(5/6)
=1
a,b为锐角
a+b=45°