sin2a=3/5所以tan2a=3/4则tan(a+b)=tan(2a-a+b)=tan[2a-(a-b)]=...
sin2a=3/5(π/4<a<π/2);则tan2a=-3/4(注意cos2a为负的);tan(a+b)=tan[(b-a)+2a]=...