Mathematics

Problem

The range of

k

for which the inequality

k \cos^2 x - k \cos x + 1 \ge 0 \forall x \in (-\infty, \infty)

is: (a)

k > -\frac{1}{2}

(b)

k > 4

(c)

-\frac{1}{2} \le k \le 4

(d)

\frac{1}{2} \le k \le 5