How to do this in C#. PLS help

Knock it off. We're not going to do your damn homework so quit posting your questions.

"Coding", in general, has got to represent one of the largest chunks of contribution to the internet (I'd guess not far behind news, and "news")--so much so that I've made small programs on my own, solely from spending some time searching various topics. In fact, there's so much out there that if you google "get area from radius c#", you'll get several code examples. A2k is not Stack Overflow however, so if you must ask questions, there are more fitting places out there.