The sum of the polynomials mn^3 and mn^4 can be found by combining like terms. In this case, both terms share the common variable mn, with differing powers of n.
To sum them up, we simply write them together as:
mn^3 + mn^4
To simplify this expression, we can factor out the common term mn^3:
mn^3(1 + n)
So, the sum of the polynomials mn^3 and mn^4 is mn^3(1 + n).