Find the value of “n” where 512 + 1152 + 864 + 24n = (8 + 6)3.
2024
Find the value of “n” where 512 + 1152 + 864 + 24n = (8 + 6)3.
- A.
9
- B.
6
- C.
2
- D.
1
Attempted by 4 students.
Show answer & explanation
Correct answer: A
Concept: The binomial cube identity (a + b)3 = a3 + 3a2b + 3ab2 + b3 expands the cube of a sum into four terms — the two cubes of the individual numbers and two middle terms carrying a factor of 3. Any numeric expansion written in this exact pattern can be matched term by term against a and b to find an unknown coefficient.
Application: Take a = 8 and b = 6, matching 512 + 1152 + 864 + 24n against the identity's four terms.
a3 = 83 = 512, matching the first term.
3a2b = 3 × 82 × 6 = 1152, matching the second term.
3ab2 = 3 × 8 × 62 = 864, matching the third term.
b3 = 63 = 216, which the expression represents as 24n — so 24n = 216.
Solving, n = 216 ÷ 24 = 9.
Cross-check: (8 + 6)3 = 143 = 2744, and 512 + 1152 + 864 + 216 = 2744 — the two independently computed totals agree, confirming n = 9.