Because 72 and 36 \(⋮\)2 and 4 so we have 2 cases :

Case 1: 72 and 36 \(⋮4\)

We have 36:4=9 

->72:9=\(8\) =>\(\dfrac{1}{8}\) (elminate)

Case 2: 72 and 36\(⋮2\)

We have :36:2=18

->72:18=4=>\(\dfrac{1}{4}\) (possible)

=>Cos 72⁰ x Cos 36⁰ =\(\dfrac{1}{4}\)

This is geometry not algebraic men :) 

Give you a :)