The answer will depend on what you mean by "do the sum". Why not simply add them together?
For example 93 = 729 and 103 = 1000 so 93 + 103 = 729 + 1000 = 1729.
NB: Totally irrelevant to the question, but the above example is the smallest integer that can be expressed as a sum of two positive cubes in two different ways - as Ramanujan was to inform the Cambridge (UK) mathematician Hardy.
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(2x - 5y)(4x^2 + 10xy + 25y^2)
There is a formula for the sum of cubes. In this case, it's (b + 1)(b^2 - b + 1)
a3 + b3 = (a + b)(a2 - ab + b2) a3 - b3 = (a - b)(a2 + ab + b2)
factor out the common monomial, 5. 5(r^3-1) Factor as a difference of cubes. 5(r-1)(r^2+r+1)
The difference of cubes has a formula. (4x - y)(16x2 + 4xy + y2)