Answer
1
Work Step by Step
We can find the cosine of coterminal angle that belongs to [$0^{\circ}$,$360^{\circ}$) for convenience.
$\cos(1800^{\circ})= \cos(1800^{\circ}-5\cdot360^{\circ})$$=\cos 0^{\circ}$
The terminal side of $0^{\circ}$ angle passes through (1,0). So $x=1,y=0$ and $r=1$.
$\cos 0^{\circ}=\frac{x}{r}=\frac{1}{1}=1$
