Use a half-angle formula.

I can never remember the formula for tan(a/2), so I do it this way:

sin(a/2) = (+/-)sqrt{1 - cos(a)}/sqrt{2}

cos(a/2) = (+/-)sqrt{1 + cos(a)}/sqrt{2}

(where we get the + or - from which quadrant the angle is in.)

So

tan(a/2) = sin(a/2)/cos(a/2) = (+/-)sqrt{1 - cos(a)}/sqrt{1 + cos(a)}

In this case, a = 30:

tan(15) = sqrt{1 - cos(30)}/sqrt{1 + cos(30)}

= sqrt{1 - sqrt{3}/2}/sqrt{1 + sqrt{3}/2}

= sqrt{2 - sqrt{3}}/sqrt{2 + sqrt{3}}

You will want to rationalize this, so multiply the numerator and denominator by sqrt{2 - sqrt{3}}. I'll just give you the answer:

tan(15) = sqrt{7 - 4sqrt{3}}

-Dan