title: Math.Abs(T value) generics c#
body: <p>I would like to create generic function Math.Abs() but i don't know how to make this.
  if i create a class Math i can't create method Abs for T value because i don't know the type of T.</p>
  
  <p>If someone know how to do this ?</p>
  
  <p>Thanks</p>