How do you solve $2cos^2theta−cos(theta)−1=0$ ?

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Answer 1 · 2016-04-25T21:19:26

Looking past the $cos$ 's, we can see that this is a factorable trinomial.

$2cos^2theta - 2costheta + costheta - 1= 0$

$2costheta(costheta - 1) + 1(costheta - 1) = 0$

$(2costheta + 1)(costheta - 1) = 0$

$costheta = -1/2 and costheta = 1$

$theta = 120˚, 240˚ and 0˚$

Hopefully this helps!

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