For cos x cos 3x - sin x sin 3x=0 use an addition or subtraction formula to simplify the equation and then find all solutions of the equation in the interval [0, pi) the answer is x1= x2= x3= x4= with x1<x2<x3<x4 11/20/2013 | Dalia from Pleasantville, IA | 1 Answer | 0 Votes Mark favorite Subscribe Comment
cos x cos 3x - sin x sin 3x = cos(x+3x) = cos(4x) Solve cos(4x) = 0 in [0, pi], 4x = k*pi+pi/2. Picking k = 0, 1, 2, 3, you have x = pi/8, 3pi/8, 5pi/8, 7pi/8. 11/20/2013 | Robert J. Comment