What is the minimum sum of the numbers if the product of 2 numbers is 81?

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The product of two numbers is 81. If one of the numbers is 81, the other number is 81/x.

The sum of the numbers is S = x + 81/x. To find the minimum value of S, S' = 0 has to be solved for x.

S' = 1 - 81/x^2

1 - 81/x^2 = 0

=> x^2 = 81

=> x = 9 and x = -9

*An assumption has to be made that the numbers are positive to be able to determine the minimum sum.*

**The minimum sum of the numbers is 18.**

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