wich of the following are valid?

Question

5x^2 + 5x^3
5x^2-5x^3
5x^2*5x^3
5x^2/5x^3

Answer 1

here is a quick test
set x=1

your equations evaluate to:
5*x^2 + 5*x^3=10
5*x^2-5*x^3 = 0
5*x^2*5*x^3 = 25
5*x^2/5*x^3=1

set x=0
(5*x^2)/(5*x^3)=0/0

which is indeterminate

so the question you have to ask yourself is "for which values of x are the following valid"?

Answer 2

These are all valid expressions.

Answer 3

They are all valid expressions.