In aptitude (algebraic identities), if a + b + c = 0, then using the identity for the sum of cubes and its square, find the value of (a^3 + b^3 + c^3)^2 in terms of a, b, and c.

Difficulty: Medium

Correct Answer: 9a^2b^2c^2

Explanation:


Introduction / Context:
This question tests understanding of algebraic identities involving the sum of cubes and the special case where a + b + c = 0. These identities are widely used in polynomial simplification, factorisation, and competitive exam problems. The key idea is that when the sum of three numbers is zero, the expression a^3 + b^3 + c^3 simplifies significantly. Here, we are asked to find the square of that simplified sum, (a^3 + b^3 + c^3)^2, purely in symbolic form, without substituting any numerical values.


Given Data / Assumptions:
- a, b, and c are real numbers.
- a + b + c = 0.
- We need to evaluate (a^3 + b^3 + c^3)^2.
- Standard algebraic identities for sums of cubes can be used.


Concept / Approach:
The fundamental identity used is:
a^3 + b^3 + c^3 - 3abc = (a + b + c)(a^2 + b^2 + c^2 - ab - bc - ca).
When a + b + c = 0, the right-hand side becomes zero, leading to a^3 + b^3 + c^3 - 3abc = 0. This simplifies to a^3 + b^3 + c^3 = 3abc. After this, we are asked to find (a^3 + b^3 + c^3)^2, which becomes (3abc)^2. This is a direct application of squaring a product.


Step-by-Step Solution:
Step 1: Recall the identity a^3 + b^3 + c^3 - 3abc = (a + b + c)(a^2 + b^2 + c^2 - ab - bc - ca).Step 2: Substitute the given condition a + b + c = 0 into the right-hand side.Step 3: Then the right-hand side becomes 0 * (a^2 + b^2 + c^2 - ab - bc - ca) = 0.Step 4: So we get a^3 + b^3 + c^3 - 3abc = 0.Step 5: Rearranging gives a^3 + b^3 + c^3 = 3abc.Step 6: Now we need (a^3 + b^3 + c^3)^2, which is (3abc)^2.Step 7: (3abc)^2 = 9a^2b^2c^2.


Verification / Alternative check:
We can verify using specific values that satisfy a + b + c = 0. For example, choose a = 1, b = 1, c = -2. Then a + b + c = 0 holds. Compute a^3 + b^3 + c^3 = 1^3 + 1^3 + (-2)^3 = 1 + 1 - 8 = -6. Then (a^3 + b^3 + c^3)^2 = (-6)^2 = 36. Now compute 9a^2b^2c^2 = 9 * 1^2 * 1^2 * (-2)^2 = 9 * 4 = 36. Both values match, confirming the identity and the final expression 9a^2b^2c^2.


Why Other Options Are Wrong:
- 27abc, 9abc, 3abc: These expressions do not match the required squared form and lack the correct exponents on a, b, and c. They represent linear or cubic products, not a squared quantity derived from 3abc.

- 27a^2b^2c^2: This would arise from incorrectly assuming a^3 + b^3 + c^3 = 3abc and then squaring it as (3abc)^3 or making a multiplication mistake with 3^3 instead of 3^2. The coefficient must be 9, not 27.


Common Pitfalls:
Students often misremember the identity and write a^3 + b^3 + c^3 = 3abc directly without stating the condition a + b + c = 0, which is essential. Another common error is to square 3abc incorrectly as 27a^2b^2c^2 instead of 9a^2b^2c^2. Careful handling of exponents and coefficients is important. Also, ignoring the given condition and trying to expand cubes individually wastes time and introduces unnecessary complexity.


Final Answer:
Under the condition a + b + c = 0, the value of (a^3 + b^3 + c^3)^2 is 9a^2b^2c^2.

More Questions from Simplification

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion