Answer:
Ф = 2pi/3 , 4pi/3
Explanation:
1 + cos Ф = 1/2
Subtract 1 from each side
1 + cos Ф -1 = 1/2-1
cos Ф = -1/2
Take the inverse cos of each side
cos^-1 ( cos Ф )= cos^-1(-1/2)
There are two solutions for the inverse cos of (-1/2), 2pi/3 and 4pi/3
Ф = 2pi/3 +2 npi
Ф = 4pi/3 + 2n pi where n is an integer
We are limited to the interval between 0 and 2pi
Ф = 2pi/3 , 4pi/3