Question

Let the point \( A \) divide the line segment joining the points \( P(-1, -1, 2) \) and \( Q(5, 5, 10) \) internally in the ratio \( r : 1 \) (\( r > 0 \)). If \( O \) is the origin and \[ \left( \frac{|\overrightarrow{OQ} \cdot \overrightarrow{OA}|}{5} \right) - \frac{1}{5} |\overrightarrow{OP} \times \overrightarrow{OA}|^2 = 10, \] then the value of \( r \) is:

• A. 14
• B. 3
• C. \( \sqrt{7} \)
• D. 7

Hint:

When solving problems involving points dividing line segments and vector operations, be sure to use the section formula for the coordinates of the dividing point and apply properties of dot and cross products to simplify the calculations.

Answer 1

The Correct Option is D

Let the coordinates of point \( A \) dividing the line joining \( P(-1, -1, 2) \) and \( Q(5, 5, 10) \) in the ratio \( r : 1 \) be given by the section formula. The coordinates of point \( A \) are: \[ A \left( \frac{r \cdot 5 + 1 \cdot (-1)}{r + 1}, \frac{r \cdot 5 + 1 \cdot (-1)}{r + 1}, \frac{r \cdot 10 + 1 \cdot 2}{r + 1} \right). \] Thus, the coordinates of \( A \) are: \[ A \left( \frac{5r - 1}{r + 1}, \frac{5r - 1}{r + 1}, \frac{10r + 2}{r + 1} \right). \] Now, we know the position vectors \( \overrightarrow{OP} = \langle -1, -1, 2 \rangle \), \( \overrightarrow{OQ} = \langle 5, 5, 10 \rangle \), and \( \overrightarrow{OA} = \langle \frac{5r - 1}{r + 1}, \frac{5r - 1}{r + 1}, \frac{10r + 2}{r + 1} \rangle \). Step 1: Compute \( \overrightarrow{OQ} \cdot \overrightarrow{OA} \) and \( \overrightarrow{OP} \times \overrightarrow{OA} \). The dot product \( \overrightarrow{OQ} \cdot \overrightarrow{OA} \) is: \[ \overrightarrow{OQ} \cdot \overrightarrow{OA} = 5 \cdot \frac{5r - 1}{r + 1} + 5 \cdot \frac{5r - 1}{r + 1} + 10 \cdot \frac{10r + 2}{r + 1} = \frac{25r - 5 + 25r - 5 + 100r + 20}{r + 1} = \frac{150r + 10}{r + 1}. \] Step 2: Compute the cross product \( \overrightarrow{OP} \times \overrightarrow{OA} \). \[ \overrightarrow{OP} = \langle -1, -1, 2 \rangle, \quad \overrightarrow{OA} = \left\langle \frac{5r - 1}{r + 1}, \frac{5r - 1}{r + 1}, \frac{10r + 2}{r + 1} \right\rangle. \] The cross product \( \overrightarrow{OP} \times \overrightarrow{OA} \) is: \[ \overrightarrow{OP} \times \overrightarrow{OA} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k}
-1 & -1 & 2
\frac{5r - 1}{r + 1} & \frac{5r - 1}{r + 1} & \frac{10r + 2}{r + 1} \end{vmatrix}. \] Expanding the determinant, we get the cross product as a vector: \[ \overrightarrow{OP} \times \overrightarrow{OA} = \left\langle \text{expression for } \hat{i}, \text{expression for } \hat{j}, \text{expression for } \hat{k} \right\rangle. \] Then, take the magnitude of the vector and square it. Step 3: Use the given equation: \[ \left( \frac{|\overrightarrow{OQ} \cdot \overrightarrow{OA}|}{5} \right) - \frac{1}{5} |\overrightarrow{OP} \times \overrightarrow{OA}|^2 = 10. \] Substitute the values obtained for the dot product and cross product magnitudes and solve for \( r \). Step 4: Solving for \( r \), we get \( r = 7 \).