tan(a-b)=1/2 tan(2a-2b)=2tan(a-b)/(1-tan²(a-b))=1/(1-1/4)=4/3 tan b=-1/7 tan(2a-b) =tan(2a-2b b) =tan(2a-2b) tanb / [1-tan(2a-2b)tanb] =(4/3-1/7)/(1 4/21) =(28-3)/(21 4) =25/25 =1
