# Thread: Trig question: with intervals

1. ## Trig question: with intervals

Solve the equation for a in [0, 2pie]

A.) cos²(a) + 3cos(a) = -cos(a)

B.)sin(2a) = cos(a)

2. Originally Posted by VkL
Solve the equation for a in [0, 2pie]

A.) cos²(a) + 3cos(a) = -cos(a)
Add $\cos a$ to both sides to get $\cos^2 a + 4\cos a = 0$ and now $\cos a ( \cos a + 4) = 0$. Therefore, $\cos a = 0$ or $\cos a + 4 = 0$. The second condition, $\cos a + 4 = 0 \implies \cos a = -4$ is impossible because $|\cos a| \leq 1$. The first condition is $\cos a = 0$. That happens when $a = \tfrac{\pi}{2},\tfrac{3\pi}{2}$

B.)sin(2a) = cos(a)
Using the double angle identity we get $2\sin a \cos a = \cos a \implies 2\sin a \cos a - \cos a = 0$. Now factor, $\cos a ( 2\sin a - 1) = 0$. Thus, we get two possibilities, $\cos a = 0$ or $2\sin a - 1 = 0$. The first condition, $\cos a = 0$, gives $a = \tfrac{\pi}{2}, \tfrac{3\pi}{2}$. The second condition, $2\sin a - 1 = 0$ is equivalent to saying $\sin a = \tfrac{1}{2}$, thus, $a = \tfrac{\pi}{6}, \tfrac{5\pi}{6}$.