Question

Question #b5428

Answer 1

`-pi; pi`

Explanation:

Solve this quadratic equation for cos x.
`cos^2 x - cos x - 2 = 0`
Since a - b + c = 0, use shortcut. The 2 real roots are: cos x = - 1, and
`cos x = -c/a = 2`
The answer cos x = 2 is rejected since > 1.
cos x = - 1 --> `x = pi + 2kpi`
In the interval `[-2pi, 0]`, with k = - 1, the answer is `- pi`
In the interval `[0, 2pi]` with k = 1, the answer is `pi`