Cos (sin15/17 - (sin-5/13)

If sin

=5/13

then the ratio of opposite side/hypotenuse

=5/13

By Pythagorean Theorem

If opposite side = 5units and

hypotenuse = 13 units

(for any units)

then adjacent side=12 units

and

cos=adjacent side over

hypotenuse = 12/13

However, we need to note that if the angle is in Quadrant II then the

adjacent side

will actually be a negative value

so

cos = 12/13 for an angle in Q1

or

cos = - 12/13 for an angle in Q2

(angle can't be in Q3 or Q4, since sin > 0)