i want to get maximum area of square can be drawn in a circle if its diamemter is "d"
eg:
7 ---->24.5
12 ---->72
29 ---->420.5
If I understand correctly the diameter will also be the longest diagonal across the square.
From Pythagoras you know that $\displaystyle d^2 = 2a^2 \text{ and } d = a\sqrt{2} \implies a = \dfrac{d}{\sqrt{2}}$ where a is the side length.
(nb: because side length must be greater than 0 I have discarded the negative root)
Now what do you know about the area of a square?