tan(a-b)=1/2tan(2a-2b)=2tan(a-b)/(1-tan²(a-b))=1/(1-1/4)=4/3tan b=-1/7tan(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