Ok, so you need to:

prompt for the radius
do some basic maths
output the result.

Nearly all of which would be included in any basic c++ tutorial online (and, at a guess, in the class you were given the assignment in?).

I know it been said before, and at the risk of sounding like a broken record, google is your friend. It's very unlikely you'll find anyone here to do your assignments for you.

Thanx but i dnt knw how 2 use that search is not goin

javascript needs to be enabled for the let me google that for you site... perhaps he has it disabled...

or perhaps he's a troll.....

Chan gave you all the logic you need. The class you are in should have at least tought you the basics for getting input and ginning up some output. Do what I do and try reading the book.