If a is proportional to b then a = kb for some constant k. (lets consider only a,b,k nonzero).

Then a²-b² = (kb)² - b² = (k²-1)b²

and ab = (kb)b=kb².

Thus (a² - b²) / ab = (k²-1)b²/(kb²) = (k²-1)/k which is a constant,
i.e. a² - b² = ab * (k²-1)/k, i.e. a²-b² is equal to ab times a constant.

Thus if a is proportional to b then a²-b² is proportional to ab (for b not equal to zero).

Thx,
ms. Dame

Hello Nazrul,
If ‘a’ is proportional to ‘b’ then  a = kb, for some number ‘k’.  So,

Let m = (k² - 1)/k then a² - b² = m ab.

Thus a² - b² is proportional to ab.

Tyler