why can you NOT combine 3x and 5x^2?
x and x^2 are different animals
you cannot combine x and y, either, or apples and oranges.
If x is a dimension, like length, then x^2 is an area. They don't combine at all.
you cannot combine x and y, either, or apples and oranges.
If x is a dimension, like length, then x^2 is an area. They don't combine at all.