tan2a=tan(a+b+a-b)
=[tan(a+b)+tan(a-b)]/[1-tan(a+b)tan(a-b)]
=(3+2)/(1-6)
=-1
∴2a=kπ-π/4
a=kπ/2-π/8
∵a为锐角,
∴当k=1时,a=3π/8