If sin(θ + 18°) = cos60° (0° < θ < 90°), then the value of cos 5θ is?

Answer: Option A

$$\begin{array}{rl}& \text{sin}\left(\theta +{18}^{\circ }\right)=\cos 60^{\circ }\left(0^{\circ }<\theta <{90}^{\circ }\right)\\ & \Rightarrow \theta +18^{\circ }+60^{\circ }=90^{\circ }\\ & \Rightarrow \theta =12^{\circ }\\ & \Rightarrow \text{cos 5}\theta \\ & \Rightarrow \text{cos }60^{\circ }\\ & \Rightarrow \frac{1}{2}\end{array}$$