Find the exact value of cos (a - b) if sin a = 3
5
and cos B =
5
13
with a in quadrant III and B in quadrant IV.
cos (a - b) = ?

For problems 29 - 31 the graph of a quadratic function y=ax^2 + bx + c is shown. tell whether the discriminant of ax^2 + bx + c = 0 is positive, negative, or zero.