Question

Find the value of cos210°

Answer 1

@KeithAfasCalcLover

Answer 2

We know that:
\[cos(x)=-cos(x-180^\circ)\]
So:
\[cos(210^\circ)=-cos(210^\circ-180^\circ)=-cos(30^\circ)\]
We need the special triangle:

So therefore:
\[cos(210^\circ)=-cos(30^\circ)=-\left(\frac{\sqrt{3}}{2}\right)=-\sqrt{1.5}\]