We need to solve the equation
cos x + 1 = 0
I order to do so, let's subtract 1 from both sides of the equation:
cos x + 1 - 1 = 0 - 1
cos x = -1
Now, the solution to this equation is the value of x for which cos x equals - 1. Since
cos π = -1
we have x = π.
Therefore, the answer is
a) pi