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Show that the triangle with vertices at \( P(-5,6), Q(2,3) \) and \( R(5,10) \) is a right-angled triangle.

Ask by Horton Nichols. in Singapore
Mar 10,2025

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Answer

The triangle with vertices at \( P(-5,6) \), \( Q(2,3) \), and \( R(5,10) \) is a right-angled triangle because the vectors \( \vec{PQ} \) and \( \vec{QR} \) are perpendicular, making the angle at vertex \( Q \) a right angle.

Solution

1. **Calculate vectors \(\vec{PQ}\) and \(\vec{QR}\):** \[ \vec{PQ} = Q - P = (2 - (-5),\; 3 - 6) = (7,\; -3) \] \[ \vec{QR} = R - Q = (5 - 2,\; 10 - 3) = (3,\; 7) \] 2. **Compute the dot product of \(\vec{PQ}\) and \(\vec{QR}\):** \[ \vec{PQ} \cdot \vec{QR} = (7)(3) + (-3)(7) = 21 - 21 = 0 \] 3. **Conclude the triangle is right-angled:** Since the dot product is zero, the vectors \(\vec{PQ}\) and \(\vec{QR}\) are perpendicular. Thus, the angle at vertex \(Q\) is a right angle, and the triangle is a right-angled triangle.

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The Deep Dive

To show that the triangle \( PQR \) is a right-angled triangle, we will check the slopes of the sides formed by the vertices and verify if they are perpendicular, indicating a right angle. 1. Calculate the slopes: - Slope of \( PQ \): \[ m_{PQ} = \frac{y_2 - y_1}{x_2 - x_1} = \frac{3 - 6}{2 - (-5)} = \frac{-3}{7} \] - Slope of \( QR \): \[ m_{QR} = \frac{10 - 3}{5 - 2} = \frac{7}{3} \] - Slope of \( RP \): \[ m_{RP} = \frac{6 - 10}{-5 - 5} = \frac{-4}{-10} = \frac{2}{5} \] 2. Check if any pairs of slopes are negative reciprocals: - For \( PQ \) and \( QR \): \[ m_{PQ} \times m_{QR} = \left( -\frac{3}{7} \right) \times \left( \frac{7}{3} \right) = -1 \quad \text{(perpendicular)} \] - For \( QR \) and \( RP \): \[ m_{QR} \times m_{RP} = \left( \frac{7}{3} \right) \times \left( \frac{2}{5} \right) = \frac{14}{15} \quad \text{(not perpendicular)} \] - For \( RP \) and \( PQ \): \[ m_{RP} \times m_{PQ} = \left( \frac{2}{5} \right) \times \left( -\frac{3}{7} \right) = -\frac{6}{35} \quad \text{(not perpendicular)} \] Since \( PQ \) and \( QR \) are perpendicular, triangle \( PQR \) is a right-angled triangle with the right angle at vertex \( Q \). Thus, we have shown that triangle \( PQR \) is indeed a right-angled triangle.

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