Changed math helper class to use its own Pi property

Maybe nitpicking, but I just wanted to say I contributed to the .NET core :)

The math helper class was re evaluating ```(float)Math.PI``` instead of using its own public property.
Also fixed the grammar on a comment while I was oggling the code.
Hmm, I'm reading this sentence as `Gets the comparer used to <obtain hash codes for the keys> and <check equality>`, so the original version is right for me (i.e. if you leave out the first part it is `Gets the comparer used to check equality`).