We are asked to find the value of cos (4/3). To do this, we can sketch it on a Cartesian plan to know which quadrant it is located. This will tell us the correct sign (positive or negative). We will also know what reference angle to use. The reference angle will tell us the magnitude or the length of the side.
So, let's draw:
We see that 4/3 is in the 3rd Quadrant so cos must be negative.
Also, we see that the reference angle is /3 or 60°.
[tex]\cos\frac{\pi}{3}=\frac{1}{2}[/tex]
So, cos (4/3) = - 1/2.
