@Sanjeev kumar Sen
Every Square is a rectangle but not every rectangle is a square
But some rectangles can be square
Consider PQRS as the rectangle initially
Let $l$ and $b$ be the length and breadth of rectangle PQRS, then its area is $l*b$
Given that $l*b$ is maximum,hence differnce between $l $ and $b$ is minimum
As $l,b$ are non-negative, the minimum difference will be 0. i..e $l-b=0$ or $l=b$
since $l=b$, the given rectangle PQRS become a square.