cot is the cotangent which is calculated by dividing cos by sen. So the dominator(15) is sen and the other number is the cos. Notice that the number is negative which means one of them (sen or cos) is negative. We know that the angule is in the forth quadrant, and that tells us that, in that quadrant, x is a positive number while y is negative. X is the cos function and Y is the sen. So, if y is negative, so is sen.
