Posted by **Thanh** on Tuesday, May 10, 2011 at 10:32pm.

Cos(2theta)=-1/2

There are 6 solutions, how do i solve this?

- Trig -
**Reiny**, Tuesday, May 10, 2011 at 10:49pm
cos (2Ø) = -1/2

we know cos 60° = + 1/2

and we know that 2Ø, our angle, must be in quadrants II or III by the CAST rule

so 2Ø = 180-60 = 120° or 2Ø = 180+60 = 240°

Ø = 60° or Ø = 120°

but the period of cos 2Ø is 180°

so adding 180° to our answers will produce more answers.

Ø = 60+180 = 240° or

Ø = 120+180 = 300°

so Ø= 60,120,240,300°

test by doubling any of these answers and taking the cosine on your calculator.

your statement that there are 6 solutions is not really correct, there is an infinite number of solutions, we can keep adding/subtracting 180 to any of our new answers to obtain more answers.

There are 6 solutions in the usual domain between 0° and 360°

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