Here the straight line AB is divided into AC and CB of measure ‘a’ and ‘b’ respectively.
we say that ‘a’ and ‘b’ are in the golden ratio, if
the ratio of the whole sum to the larger measure equals to the ratio of the larger measure to the smaller measure.
Φ=(a+b)/a =a/b
The value of Φ is$ (1+√5)/2 =1.618…$