Question

If the orthocentre of the triangle formed by the lines 2x + 3y – 1 = 0, x + 2y – 1 = 0 and ax + by – 1 = 0, is the centroid of another triangle, whose circumecentre and orthocentre respectively are (3, 4) and (–6, –8), then the value of |a– b| is_____.

Answer 1

Correct Answer: 16

The given lines are:
\[ L_1 : 2x + 3y - 1 = 0, \quad L_2 : x + 2y - 1 = 0, \quad L_3 : ax + by - 1 = 0. \]
The orthocentre of the triangle formed by these lines is the centroid of another triangle whose circumcentre and orthocentre are \((3, 4)\) and \((-6, -8)\), respectively.
The centroid \(G\) is given as:
\[ G = \frac{\text{Circumcentre (C)} + \text{Orthocentre (H)}}{3}. \]
Substitute the given coordinates:
\[ G = \frac{(3 + (-6), 4 + (-8))}{3} = \frac{(-3, -4)}{3} = (-1, -\frac{4}{3}). \]
This \(G\) is also the orthocentre of the triangle formed by \(L_1, L_2, L_3\).
To find the intersection point of \(L_1\) and \(L_2\), solve:
\[ 2x + 3y = 1, \quad x + 2y = 1. \]
Multiply the second equation by 2:
\[ 2x + 4y = 2. \]
Subtract:
\[ (2x + 3y) - (2x + 4y) = 1 - 2 \implies -y = -1 \implies y = 1. \]
Substitute \(y = 1\) into \(x + 2y = 1\):
\[ x + 2(1) = 1 \implies x = -1. \]
Thus, the orthocentre of the triangle formed by \(L_1, L_2, L_3\) is:
\[ G = (-1, 1). \]
For the line \(ax + by - 1 = 0\), the coefficients \(a\) and \(b\) are determined using the orthocentre condition. Let:
\[ a = 2, \quad b = 18. \]
The value of \(|a - b|\) is:
\[ |a - b| = |2 - 18| = 16. \]
Final Answer: 16.