Golden Ratio

The golden ratio is a number, commonly known as phi ($\varphi$), defined as $\varphi ^{2}=\varphi +1$. The value of phi is

${\frac {1+{\sqrt {5}}}{2}}$

Phi is an irrational number. In decimal form it is often written

1.618...

It has been observed that the golden ratio is evident in the arrangement of branches along the stems of plants, and of veins in leaves.