please help following questions for Unless otherwise specif

please help!

following questions for . Unless otherwise specified, give all solutions as Student Number res such that 0 measures such that

20) 2cos² -5cos + 10 = 13

==> 2cos² -5cos + 10 - 13 = 0

==> 2cos² -5cos - 3 = 0

==> 2cos² -6cos + cos - 3 = 0

==> 2cos(cos -3) + 1(cos -3) = 0

==> (cos -3)(2cos +1) = 0

==> cos = 3 , -1/2

since the range of cos is [-1 , 1] , 3 cannot be solution

Hence cos = -1/2

==> = 2/3 radians