My double is two away from me What am I?

Formerly associated with the labor of the ancestors, Now I'm mostly known as a shape, where the higher part of me gets smaller. My volume is one-third my base area multiplied by how tall I am.

I make up most of the universe yet you cannot see me I'm everywhere and nowhere yet you can't touch me I'm what looks like a shadow and known by all If i ceased to exist life would stall What am I

My maths teacher told me that altering an integral relating to a triangle was wrong. What did that leave me doing?

In English, but also in Math, 'Eleven plus two' Equals?