Kenneth S. answered • 02/05/18

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Expert Help in Algebra/Trig/(Pre)calculus to Guarantee Success in 2018

from the givens, u is in quadrant IV and the reference triangle has the well known 5-12-13 shape (one of the Pythag. triples that you should have in your memory).

so now you know the cos u (it's positive 5/13, right?) and the half angle formula and the douible angle formulas can be used to compute the answers wanted in a, b, c.