SOLUTION: Write expressions that give all solutions to the following equation. (Express your answers in degrees. Let k be any integer. Enter your answers as a comma-separated list.) cos &#9

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Question 1016640: Write expressions that give all solutions to the following equation. (Express your answers in degrees. Let k be any integer. Enter your answers as a comma-separated list.)
cos θ/2 − cos θ = 1
Note: I have tried using the half-angle formal and squared boy sides to get rid of the square root. After that i found common denominators and did the algebraic stuff. I got 60, 180, and 300. I let k be an integer of 360k for each, but it was wrong. Thank you!

Answer by ikleyn(52905) About Me  (Show Source):
You can put this solution on YOUR website!
.
Write expressions that give all solutions to the following equation. (Express your answers in degrees. Let k be any integer. Enter your answers as a comma-separated list.)
cos θ/2 − cos θ = 1
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cos%28theta%2F2%29+-+cos%28theta%29 = 1.    (1)

For me, it is easier to introduce new variable alpha = theta%2F2 and to solve an equation for alpha

cos%28alpha%29 - cos%282%2Aalpha%29 = 1.   (2)

Surely, you know that cos%282%2Aalpha%29 = cos%5E2%28alpha%29+-+sin%5E2%28alpha%29 = 2%2Acos%5E2%28alpha%29+-+1. 

Substitute it into (2). You will get

cos%28alpha%29+-+2%2Acos%5E2%28alpha%29+%2B+1 = 1,   or

cos%28alpha%29+-+2%2Acos%5E2%28alpha%29 = 0,

cos%28alpha%29%2A%281+-+2%2Acos%28alpha%29%29 = 0.

Thus you have two separate equations 

1.  cos%28alpha%29 = 0  --->  alpha = pi%2F2+%2B+k%2Api  --->  theta = pi+%2B+2%2Ak%2Api, k = 0, +/-1, +/-2, . . . 

2.  2%2Acos%28alpha%29 = 1  --->  cos%28alpha%29 = 1%2F2  --->  alpha = +/- pi%2F3 + 2k%2Api   --->  theta = +/-2pi%2F3 + 2k%2Api, k = 0, +/-1, +/-2, . . .