Find the value of

To find: Value of cos 2220°

We have,

$\cos \left(-2220^{\circ}\right)=\cos 2220^{\circ}$

${[\because \cos (-\theta)=\cos \theta] }$

$=\cos \left[2160+60^{\circ}\right]$

$=\cos \left[360^{\circ} \times 6+60^{\circ}\right]$

$=\cos 60^{\circ}$

[Clearly, $2220^{\circ}$ is in I $^{\text {st }}$ Quadrant and the multiple of $360^{\circ}$ is even]

$=\frac{1}{2}\left[\because \cos 60^{\circ}=\frac{1}{2}\right]$