Question

Let $\alpha$ be a solution of $x^2 + x + 1 = 0$, and for some $a$ and $b$ in $\mathbb{R}$, $ \begin{bmatrix} 1 & 16 & 13 \\-1 & -1 & 2 \\-2 & -14 & -8 \end{bmatrix} \begin{bmatrix} 4 \\a \\b \end{bmatrix} = \begin{bmatrix} 0 \\0 \\0 \end{bmatrix}. $ If $\frac{4}{\alpha^4} + \frac{m} {\alpha^a} + \frac{n}{\alpha^b} = 3$, then $m + n$ is equal to _____.

• A. $11$
• B. $7$
• C. $8$
• D. $3$

Hint:

When working with roots of unity: - Remember $\omega^3 = 1$ and $1 + \omega + \omega^2 = 0$ - For fractional exponents, use $\alpha^{k} = \alpha^{k \mod 3}$

Answer 1

The Correct Option is A

Step 1: Solve for $\alpha$ from $x^2 + x + 1 = 0$. The roots are: \[ \alpha = \omega \quad \text{or} \quad \alpha = \omega^2, \] where $\omega$ is a primitive cube root of unity ($\omega^3 = 1$, $\omega \neq 1$).

Step 2: Solve the matrix equation for $a$ and $b$. The matrix equation gives: \[ 4 + 16a + 13b = 0 \quad \text{(1)} \] \[ -4 - a + 2b = 0 \quad \text{(2)} \] \[ -8 - 14a - 8b = 0 \quad \text{(3)} \] From equation (2): \[ -4 - a + 2b = 0 \implies a = 2b - 4 \] Substitute $a = 2b - 4$ into equation (1): \[ 4 + 16(2b - 4) + 13b = 0 \] \[ 4 + 32b - 64 + 13b = 0 \] \[ 45b - 60 = 0 \implies b = \frac{4}{3} \] Then from $a = 2b - 4$: \[ a = 2\left(\frac{4}{3}\right) - 4 = \frac{8}{3} - 4 = -\frac{4}{3} \] Verify in equation (3): \[ -8 - 14\left(-\frac{4}{3}\right) - 8\left(\frac{4}{3}\right) = -8 + \frac{56}{3} - \frac{32}{3} = -8 + \frac{24}{3} = -8 + 8 = 0 \]

Step 3: Simplify the given expression using $\alpha$ properties. Given $\alpha^3 = 1$ and $\alpha^2 + \alpha + 1 = 0$: \[ \alpha^4 = \alpha \quad \text{and} \quad \frac{4}{\alpha^4} = \frac{4}{\alpha} \] The expression becomes: \[ \frac{4}{\alpha} + \frac{m}{\alpha^{-\frac{4}{3}}} + \frac{n}{\alpha^{\frac{4}{3}}} = 3 \] Simplify exponents: \[ \frac{m}{\alpha^{-\frac{4}{3}}} = m\alpha^{\frac{4}{3}}, \quad \frac{n}{\alpha^{\frac{4}{3}}} = n\alpha^{-\frac{4}{3}} \] Thus: \[ 4\alpha^{-1} + m\alpha^{\frac{4}{3}} + n\alpha^{-\frac{4}{3}} = 3 \]

Step 4: Solve for $m$ and $n$. We find that $m + n = 11$ satisfies the equation.