Difficulty: Medium
Correct Answer: 0
Explanation:
Introduction / Context:
This question is about recognizing a famous algebraic identity:
p^3 + q^3 + r^3 − 3pqr, especially when p + q + r = 0. If you spot the pattern, the expression collapses immediately.
Given Data / Assumptions:
Concept / Approach:
Let:
p = a − 7, q = b − 9, r = c − 11.
Then the expression becomes:
p^3 + q^3 + r^3 − 3pqr.
Also:
p + q + r = (a + b + c) − (7 + 9 + 11) = 27 − 27 = 0.
Identity:
If p + q + r = 0, then p^3 + q^3 + r^3 − 3pqr = 0.
Step-by-Step Solution:
1) Define p = a − 7, q = b − 9, r = c − 11
2) Compute sum: p + q + r = (a + b + c) − 27 = 27 − 27 = 0
3) Recognize identity form: p^3 + q^3 + r^3 − 3pqr
4) Apply identity for p + q + r = 0:
p^3 + q^3 + r^3 − 3pqr = 0
Verification / Alternative check:
Pick a simple example satisfying a + b + c = 27, such as a = 7, b = 9, c = 11. Then p = q = r = 0 and the expression is clearly 0. This confirms the result matches the identity behavior.
Why Other Options Are Wrong:
• 3, 9, 27, 81: these are typical cube-related distractors, but the identity forces the result to be exactly 0.
Common Pitfalls:
• Expanding cubes directly (unnecessary and error-prone).
• Not noticing that (7 + 9 + 11) equals 27, making p + q + r = 0.
Final Answer:
0
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