The product of three consecutive positive integers is times their sum. What is the sum of their squares?
- A)
- B)
- C)
- D)
- E)
Answer
B
Key insight
Call the middle number n. The sum is 3n and the product is (n-1)n(n+1), so the condition becomes (n-1)(n+1) = 24, giving n = 5.
Solution
1. Call the middle number , so the three numbers are , , . Putting the letter in the middle makes the sum simple:
2. The condition says product sum:
3. Both sides have a factor of , and is not zero, so divide it away:
4. Which two numbers that differ by multiply to ? . So and . The numbers are .
5. Check: and . Good.
6. The question asks for the sum of the squares: .
The answer is .
Why this works
With consecutive numbers, naming the middle one keeps the algebra tidy: the sum becomes , and a matching factor of cancels from both sides. Then a small "which two numbers" question finishes it. Always reread what the question asks for at the end.
Alternative approach
Guess and check small cases:
| numbers | product | sum |
|---|---|---|
The product grows much faster than the sum, so is the only match, and .
The trap
Finding the numbers 4, 5, 6 and then answering their sum, 15, instead of the sum of their squares.
Concise solution
Let the integers be , , with . Their sum is and their product is . The condition reads
Since we may divide by : , so and . The integers are .
Sum of squares: .
The answer is .
Review in the AoPS books
- Introduction to Algebra, Chapter 11: Special Factorizations (§11.2, §11.3, §11.5)
- Introduction to Number Theory, Chapter 1: Integers: The Basics
- Intermediate Algebra, Chapter 20: Some Advanced Strategies (§20.2)
- Introduction to Algebra, Chapter 22: Special Manipulations
Techniques
Set up the equation/formula and compute; no special trick needed · Substitute to simplify (u = x+1/x, shifting, scaling)