You should know the sines, cosines and tangents that come from
30-60-90 right triangles and isosceles right triangles. Remember that
$\pi/6$ radians is 30 degrees, $\pi/4$ radians is 45 degrees, and
$\pi/3$ radians is 60 degrees:
$\sin(\pi/6) = 1/2;$
$\cos(\pi/6) = \sqrt{3}/2;$
$\tan(\pi/6)= \sqrt{3}/3$
$\sin(\pi/4)=\sqrt{2}/2;$
$\cos(\pi/4) = \sqrt{2}/2;$
$\tan(\pi/4)=1$
$\sin(\pi/3)=\sqrt{3}/2,$
$\cos(\pi/3)=1/2;$
$\tan(\pi/3)=\sqrt{3}$
From these values, we can figure out the trig functions of common angles
in the second, third and fourth quadrant. These are developed in the
following two videos: