tanA=3/4
tan(A+B)=(tanA+tanB)/(1-tanAtanB)=(3/4+2)/(1-3/4*2)=-11/2
tanC=tan(π-A-B)=-tan(A+B)=11/2
tan(2C)=2tanC/(1-tanCtanC)=(2*11/2)/(1-11/2*11/2)=-44/117
cosA=4/5 sinA=3/5 tanA=3/4
tanC=-tan(A+B)=-(tanA+tanB)/(1+tanAtanB)=-(11/4)/(1+3/2)=-33/10
tan2C=2tanC/(1-tan^2C)=-(33/5)/(1-33^2/10^2)
=660/989