# Find the exact value

• Apr 8th 2009, 10:37 AM
mj.alawami
Find the exact value
1) $\arccos ( \frac{-1}{2} )$

Attempt using the calculator

I am getting 120 degrees but should I take 120 degrees or the reference angle which is 60 as the answer?
• Apr 8th 2009, 11:22 AM
running-gag
Hi

$0 \leq 120 \leq 180$ therefore $Arccos\left(-\frac12\right) = 120\: deg$
• Apr 8th 2009, 11:24 AM
masters
Quote:

Originally Posted by mj.alawami
1) $\arccos ( \frac{-1}{2} )$

Attempt using the calculator

I am getting 120 degrees but should I take 120 degrees or the reference angle which is 60 as the answer?

Hi mj,

Cosine is negative in QII and QIII. Cos 120 = -1/2.

The angle in QIII would be Cos(240) = -1/2