Posted by **Greg** on Tuesday, January 14, 2014 at 10:55pm.

Mark any solutions to the equation 2cos2x - 1 = 0. The answer can be more than one. Thank you

π/8

-7π/4

15π/4

3π/4

- TRIG help quickly please -
**Reiny**, Tuesday, January 14, 2014 at 11:22pm
2 cos (2x) - 1 = 0

cos (2x) = 1/2

2x = π/3 or 2x = 2π-π/3 or 5π/3

x = π/6 or x = 5π/6

none of the given answers apply

I will assume you meant

2 cos^2 x - 1 = 0

by definition:

2cos^2 x -1 = cos (2x)

so cos(2x) = 0

2x = π/2 or 2x = 3π/2

x = π/4 or x = 3π/4

I see one of those answers. 3π/4

looking at the others, note that -7π/4 is coterminal with π/4

so it would be a solution as well

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