Question

Solve 2cos^2x + cosx − 1 = 0 for x over the interval [0, 2 π ).

Answer 1

Let cos x = a, then
2a^2 + a - 1 = 0,
solving the quadratic equation, we have:
a = 0.5 or -1.

i.e. cos x = 0.5 or cos x = -1
for cos x = 0.5,
x = pi/3, 2pi - pi/3 = pi/3, 5pi/3

for cos x = -1,
x = pi

therefore, x = pi, pi/3, 5pi/3
Answer: B