If a = 299, b = 298, and c = 297, evaluate the expression: 2a^3 + 2b^3 + 2c^3 - 6abc What is the exact value?

Difficulty: Medium

Correct Answer: 5364

Explanation:


Introduction / Context:
This problem tests use of a cube identity: a^3 + b^3 + c^3 - 3abc = (a + b + c)(a^2 + b^2 + c^2 - ab - bc - ca). The given expression is exactly 2 times that identity form. Using the identity avoids heavy cube calculations.


Given Data / Assumptions:

  • a = 299, b = 298, c = 297
  • Expression: 2a^3 + 2b^3 + 2c^3 - 6abc = 2*(a^3 + b^3 + c^3 - 3abc)


Concept / Approach:
Factor out 2, then use the identity for a^3 + b^3 + c^3 - 3abc. Because a, b, c are consecutive integers, the symmetric difference term (a^2 + b^2 + c^2 - ab - bc - ca) becomes small, making the product manageable.


Step-by-Step Solution:

Rewrite: 2a^3 + 2b^3 + 2c^3 - 6abc = 2*(a^3 + b^3 + c^3 - 3abc). Compute sum: a + b + c = 299 + 298 + 297 = 894. Compute S = a^2 + b^2 + c^2 - ab - bc - ca. Instead of expanding huge squares, use: S = (1/2)*[(a - b)^2 + (b - c)^2 + (c - a)^2]. Here a - b = 1, b - c = 1, c - a = -2. So S = (1/2)*[1^2 + 1^2 + (-2)^2] = (1/2)*(1 + 1 + 4) = (1/2)*6 = 3. Therefore a^3 + b^3 + c^3 - 3abc = (a + b + c)*S = 894*3 = 2682. Multiply by 2: required value = 2*2682 = 5364.


Verification / Alternative check:
Because the numbers are close, the difference-based formula for S is very reliable and quick. If you compute directly with cubes, you get the same 5364 but with much heavier arithmetic.


Why Other Options Are Wrong:

5154 or 5267 come from incorrect S value (often forgetting the (1/2) factor). 5456 results from sum mistake (894 mis-added) or multiplying by 2 incorrectly. 5346 is a digit transposition error.


Common Pitfalls:
Using the identity but forgetting to multiply by 2 at the end, or computing S as (a-b)^2 + (b-c)^2 + (c-a)^2 without halving.


Final Answer:
5364

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