Excuse me kuldeep don’t blame nitin sir understand if there is mistake in lecture then you can correct it but why are you saying this that “worse mistake by nitin sir” don’t forgot that mistakes are done by humans, you also do some mistakes in your life then don’t say that worse mistake. ……
I hope you understand. And next time don’t blame nitin sir.

If you asking about the area of the circle then A = (Pi)r^2
And as we know the Diameter(d) = 2r; r= d/2
when we put this value we will get:- A = (Pi)(d/2)^2 = (Pi)(d^2/4)