Question 29 of 480
Three consecutive positive integers k, l and m are such that l2 = 3(k+m). Find the value of m.
Correct Answer:
D
Explanation
To solve the problem, we need to analyze the equation given for three consecutive positive integers \( k \), \( l \), and \( m \). The relationship provided is:
\[
l^2 = 3(k + m)
\]
### Step 1: Define the integers
Since \( k \), \( l \), and \( m \) are consecutive integers, we can express them in terms of \( l \):
- Let \( l = n \) (the middle integer),
- Then \( k = n - 1 \) (the integer before),
- And \( m = n + 1 \) (the integer after).
### Step 2: Substitute into the equation
Now, substituting \( k \) and \( m \) into the equation:
\[
l^2 = 3(k + m) \implies n^2 = 3((n - 1) + (n + 1))
\]
### Step 3: Simplify the right side
The expression inside the parentheses simplifies as follows:
\[
(n - 1) + (n + 1) = n - 1 + n + 1 = 2n
\]
Thus, we can rewrite the equation as:
\[
n^2 = 3(2n)
\]
### Step 4: Solve for \( n \)
Now, we simplify the equation:
\[
n^2 = 6n
\]
Rearranging gives us:
\[
n^2 - 6n = 0
\]
Factoring out \( n \):
\[
n(n - 6) = 0
\]
This gives us two solutions:
1. \( n = 0 \) (not a positive integer)
2. \( n = 6 \) (valid solution)
### Step 5: Find \( k \), \( l \), and \( m \)
Since \( n = 6 \):
- \( l = n = 6 \)
- \( k = n - 1 = 5 \)
- \( m = n + 1 = 7 \)
### Step 6: Conclusion
The value of \( m \) is therefore:
\[
\boxed{7}
\]
### Explanation of Other Options
- **Option A (4)**: If \( m = 4 \), then \( l \) would have to be 3 and \( k \) would be 2. Substituting these values into the equation \( l^2 = 3(k + m) \) gives \( 3^2 = 3(2 + 4) \) or \( 9 = 18 \), which is false.
- **Option B (5)**: If \( m = 5 \), then \( l = 4 \) and \( k = 3 \). Substituting gives \( 4^2 = 3(3 + 5) \) or \( 16 = 24 \), which is also false.
- **Option C (6)**: If \( m = 6 \), then \( l = 5 \) and \( k = 4 \). Substituting gives \( 5^2 = 3(4 + 6) \) or \( 25 = 30 \), which is false.
### Revision Summary
- The integers \( k \), \( l \), and \( m \) are consecutive integers.
- The equation \( l^2 = 3(k + m) \) can be simplified by expressing \( k \) and \( m \) in terms of \( l \).
- The valid solution for \( l \) is 6, leading to \( m = 7 \).
- Other options do not satisfy the equation when substituted back.