ExpertsGlobal5
The number 92
can be written as the sum of cubes of three different positive integers. What is the sum of these three positive integers?
A. 6
B. 7
C. 8
D. 9
E. 12
To find the three integers, we first list the perfect cubes of positive integers that are less than 92.
Step 1: List Possible CubesWe need \(x^3 + y^3 + z^3 = 92\).
Let's list the first few cubes:
\(1^3 = 1\)
\(2^3 = 8\)
\(3^3 = 27\)
\(4^3 = 64\)
\(5^3 = 125\) (Too large, so the integers must be chosen from the set {1, 2, 3, 4})
Step 2: Find the CombinationWe need to pick three
different numbers from the set of cubes \(\{1, 8, 27, 64\}\) that add up to 92.
Let's start with the largest possible value to cover the most "ground":
Try \(64\) (which is \(4^3\)).
Remaining sum needed: \(92 - 64 = 28\).
Now we need two distinct cubes from the remaining set \(\{1, 8, 27\}\) that sum to 28.
It is easy to see that:
\(27 + 1 = 28\)
So the cubes are \(64\), \(27\), and \(1\).
Let's verify:
\(4^3 + 3^3 + 1^3 = 64 + 27 + 1 = 92\).
The three distinct positive integers are \(4\), \(3\), and \(1\).
Step 3: Calculate the SumThe question asks for the sum of these three positive integers:
\(Sum = 4 + 3 + 1 = 8\)
This matches Option (C).
Answer: C