cos(4t) = 0 =====> 4t =, where k is any integer k = 0, +/-1, +/-2, . . . ====> t = , where k is any integer k = 0, +/-1, +/-2, . . . . Answer. The given equation has infinitely many solutions t = + , where k is any integer k = 0, +/-1, +/-2, . . . .