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Question 90

If the lines $$\dfrac{x-1}{2} = \dfrac{2-y}{3} = \dfrac{z-3}{\alpha}$$ and $$\dfrac{x-4}{5} = \dfrac{y-1}{2} = \dfrac{z}{\beta}$$ intersect, then the magnitude of the minimum value of $$8\alpha\beta$$ is ______.


Correct Answer: 0

To find the minimum magnitude of $$8\alpha\beta$$:

For two lines to intersect, they must be coplanar. This is true if the determinant of the distance between points and the direction vectors is zero:

$$\begin{vmatrix} x_2-x_1 & y_2-y_1 & z_2-z_1 \\ l_1 & m_1 & n_1 \\ l_2 & m_2 & n_2 \end{vmatrix} = 0$$

Using points $$A(1, 2, 3)$$ and $$B(4, 1, 0)$$, and directions $$(2, -3, \alpha)$$ and $$(5, 2, \beta)$$:

$$\begin{vmatrix} 3 & -1 & -3 \\ 2 & -3 & \alpha \\ 5 & 2 & \beta \end{vmatrix} = 0$$

Expanding the determinant:

$$3(-3\beta - 2\alpha) + 1(2\beta - 5\alpha) - 3(4 + 15) = 0$$$$-9\beta - 6\alpha + 2\beta - 5\alpha - 57 = 0$$

$$-11\alpha - 7\beta = 57 \implies \mathbf{11\alpha + 7\beta = -57}$$

The magnitude $$|8\alpha\beta|$$ represents an absolute value, which is always $$\ge 0$$.

  • Since the relationship $$11\alpha + 7\beta = -57$$ allows for either $$\alpha$$ or $$\beta$$ to be zero (e.g., if $$\alpha = 0$$, then $$\beta = -57/7$$), the product $$8\alpha\beta$$ can equal 0.

Final Answer: The minimum magnitude is 0.

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