If a 45° and b = 15°, what is the value of $\frac{cos(a - b) - cos(a + b)}{cos(a - b) + cos(a + b)}$?

$2-2\sqrt{2}$ $3-\sqrt{6}$ $3-\sqrt{2}$ $2-\sqrt{3}$

$2-\sqrt{3}$