In a regular pentagon with side 1 the length of a diagonal equals [math]\frac{1+\sqrt{5}}{2}=\Phi[/math].[br]Luca Pacioli already knew you can find [math]\Phi[/math] in the diagonal of a regular-pentagon.[br]He notices the issue in his famous book [url=https://archive.org/details/divinaproportion00paci/mode/2up?view=theater]Divina Proportione[/url] (first version in 1498, printed version in 1509).