If cos20° = m and cos70° =n, then the value of m2 + n2 is?

Answer: Option B

$$\begin{array}{rl}& \cos 20^{\circ }=m\\ & \cos 70^{\circ }=n\\ & \text{So,}\\ & \iff m^2+n^2=\text{co}{\text{s}}^{2}20^{\circ }+\text{co}{\text{s}}^{2}70^{\circ }\\ & \left[\text{If co}{\text{s}}^2\text{A + co}{\text{s}}^2\text{B}=\text{1}\right]\\ & (\text{If, A}+\text{B}=90^{\circ })\\ & \iff 1\end{array}$$