已知tan(a+b)=3,tan(a-b)=5,求tan 2a,tan 2b的值

tan 2a=tan[(a+b)+(a-b)]
=[tan(a+b) + tan(a-b)]/[1-tan(a+b)×tan(a-b)]
=(3+5)/(1-3×5)
=-4/7
tan 2a=tan[(a+b)-(a-b)]
=[tan(a+b)- tan(a-b)]/[1+tan(a+b)×tan(a-b)]
=(3-5)/(1+3×5)
=-1/8