Given that tan 4pi/9=5.6713, determine the following, to four decimal places, without using a calculator. a)cot(pi/18) b)tan(13pi/9)

I got a by using a cofunction identity:

cot(pi/18)= tan(pi/2-pi/18)

=tan(4pi/9)

=5.6713

But I don't know how to do b. What formula do I use?