The value of cos220° + cos270° is?

Answer: Option D

$$\begin{array}{rl}& \text{co}{\text{s}}^{2}20^{\circ }+\text{co}{\text{s}}^{2}70^{\circ }\\ & =\text{co}{\text{s}}^2\left({90}^{\circ }-{70}^{\circ }\right)+\text{co}{\text{s}}^{2}70^{\circ }\\ & ={\sin }^{2}70^{\circ }+\text{co}{\text{s}}^{2}70^{\circ }\\ & =1\end{array}$$