Let 'n' be the number of side of a regular polygon each angle are equal and sum of each angle equal to (n-2)×180. ∴For a regular pentagon have five equal side, n=5 ∴ sum of each angle = (n-2)×180=3×180 = 540. and ecah angle = 540÷5=108°. Therefore ∠BCG = 108° and ∠ABC =108° Here BE is one of the diagonal of regular pentagon ABCDE. BE divide ∠ABC in 1:2 ratio Therefore ∠ABE : ∠CBE = 36: 72∴ ∠GCB + ∠CBE = 108° +72°=180°
