Algebra
Polynomials
GRB_1000_SCQ
Grade Class 12

Question:

If $\alpha$ is the root of the equation $x^2 - x + 2 = 0$ then the value of $\dfrac{6(-\alpha^3 + 2\alpha^2 - \alpha)}{\alpha^5 - 3\alpha^4 + 3\alpha^3 - \alpha^2}$ is equal to:
3
6
9
12

Step-by-Step Solution

Key Concept: Using roots of polynomial equations to simplify expressions
Step 1: Use the given equation to establish a key relationship. Since $\alpha$ is a root of $x^2 - x + 2 = 0$, we have: $$\alpha^2 - \alpha + 2 = 0$$ This gives us the important relation: $$\alpha^2 = \alpha - 2$$ Step 2: Simplify the numerator using the relation from Step 1. We need to simplify $-\alpha^3 + 2\alpha^2 - \alpha$. Factor out $-\alpha$: $$-\alpha^3 + 2\alpha^2 - \alpha = -\alpha(\alpha^2 - 2\alpha + 1)$$ Recognize that $\alpha^2 - 2\alpha + 1 = (\alpha - 1)^2$: $$-\alpha^3 + 2\alpha^2 - \alpha = -\alpha(\alpha - 1)^2$$ Step 3: Simplify the denominator using the relation from Step 1. We need to simplify $\alpha^5 - 3\alpha^4 + 3\alpha^3 - \alpha^2$. Factor out $\alpha^2$: $$\alpha^5 - 3\alpha^4 + 3\alpha^3 - \alpha^2 = \alpha^2(\alpha^3 - 3\alpha^2 + 3\alpha - 1)$$ Recognize that $\alpha^3 - 3\alpha^2 + 3\alpha - 1 = (\alpha - 1)^3$ (binomial expansion): $$\alpha^5 - 3\alpha^4 + 3\alpha^3 - \alpha^2 = \alpha^2(\alpha - 1)^3$$ Step 4: Substitute the simplified numerator and denominator into the original expression. $$\frac{6(-\alpha^3 + 2\alpha^2 - \alpha)}{\alpha^5 - 3\alpha^4 + 3\alpha^3 - \alpha^2} = \frac{6 \cdot (-\alpha)(\alpha - 1)^2}{\alpha^2(\alpha - 1)^3}$$ Step 5: Cancel common factors. Cancel $(\alpha - 1)^2$ from numerator and denominator: $$\frac{6 \cdot (-\alpha)(\alpha - 1)^2}{\alpha^2(\alpha - 1)^3} = \frac{-6\alpha}{\alpha^2(\alpha - 1)}$$ Cancel one factor of $\alpha$: $$\frac{-6\alpha}{\alpha^2(\alpha - 1)} = \frac{-6}{\alpha(\alpha - 1)}$$ Step 6: Evaluate $\alpha(\alpha - 1)$ using the relation from Step 1. $$\alpha(\alpha - 1) = \alpha^2 - \alpha$$ Substitute $\alpha^2 = \alpha - 2$: $$\alpha^2 - \alpha = (\alpha - 2) - \alpha = -2$$ Step 7: Calculate the final value. $$\frac{-6}{\alpha(\alpha - 1)} = \frac{-6}{-2} = 3$$ **Final Answer:** The value of the given expression is $\boxed{3}$, which corresponds to **Option 1**.
Correct Answer: 3

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