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.

  1. A.

    9

  2. B.

    6

  3. C.

    2

  4. 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.

  1. a3 = 83 = 512, matching the first term.

  2. 3a2b = 3 × 82 × 6 = 1152, matching the second term.

  3. 3ab2 = 3 × 8 × 62 = 864, matching the third term.

  4. b3 = 63 = 216, which the expression represents as 24n — so 24n = 216.

  5. 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.

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