We are given the line \( L: mx - y + 5m - 4 = 0 \) intersecting three other lines at points \( R, S, \) and \( T \), with distances \( r_1, r_2, \) and \( r_3 \) from the point \( (-5, -4) \). The condition to satisfy is:
\[ \left( \frac{15}{r_1} \right)^2 + \left( \frac{10}{r_2} \right)^2 = \left( \frac{6}{r_3} \right)^2 \]
Step 1: Find the intersection points \( R, S, \) and \( T \).
1. Intersection \( R \) of \( L \) and \( x + 3y + 2 = 0 \):
Solve the system:
\[ \begin{cases} mx - y + 5m - 4 = 0 \\ x + 3y + 2 = 0 \end{cases} \]
Expressing \( x \) from the second equation: \( x = -3y - 2 \). Substitute into the first equation:
\[ m(-3y - 2) - y + 5m - 4 = 0 \implies -3my - 2m - y + 5m - 4 = 0 \] \[ (-3m - 1)y + 3m - 4 = 0 \implies y = \frac{4 - 3m}{3m + 1} \]
Then, \( x = -3\left( \frac{4 - 3m}{3m + 1} \right) - 2 = \frac{-12 + 9m - 6m - 2}{3m + 1} = \frac{3m - 14}{3m + 1} \).
Thus, \( R = \left( \frac{3m - 14}{3m + 1}, \frac{4 - 3m}{3m + 1} \right) \).
2. Intersection \( S \) of \( L \) and \( 2x + 3y + 4 = 0 \):
Solve the system:
\[ \begin{cases} mx - y + 5m - 4 = 0 \\ 2x + 3y + 4 = 0 \end{cases} \]
Expressing \( y \) from the first equation: \( y = mx + 5m - 4 \). Substitute into the second equation:
\[ 2x + 3(mx + 5m - 4) + 4 = 0 \implies 2x + 3mx + 15m - 12 + 4 = 0 \] \[ (2 + 3m)x + 15m - 8 = 0 \implies x = \frac{8 - 15m}{3m + 2} \]
Then, \( y = m\left( \frac{8 - 15m}{3m + 2} \right) + 5m - 4 = \frac{8m - 15m^2 + 15m^2 + 10m - 12m - 8}{3m + 2} = \frac{6m - 8}{3m + 2} \).
Thus, \( S = \left( \frac{8 - 15m}{3m + 2}, \frac{6m - 8}{3m + 2} \right) \).
3. Intersection \( T \) of \( L \) and \( x - y - 5 = 0 \):
Solve the system:
\[ \begin{cases} mx - y + 5m - 4 = 0 \\ x - y - 5 = 0 \end{cases} \]
Subtract the second equation from the first:
\[ (m - 1)x + 5m + 1 = 0 \implies x = \frac{-5m - 1}{m - 1} \]
Then, \( y = x - 5 = \frac{-5m - 1 - 5m + 5}{m - 1} = \frac{-10m + 4}{m - 1} \).
Thus, \( T = \left( \frac{-5m - 1}{m - 1}, \frac{-10m + 4}{m - 1} \right) \).
Step 2: Compute distances \( r_1, r_2, \) and \( r_3 \) from \( (-5, -4) \).
1. Distance \( r_1 \) to \( R \):
\[ r_1 = \sqrt{ \left( \frac{3m - 14}{3m + 1} + 5 \right)^2 + \left( \frac{4 - 3m}{3m + 1} + 4 \right)^2 } \] \[ = \sqrt{ \left( \frac{3m - 14 + 15m + 5}{3m + 1} \right)^2 + \left( \frac{4 - 3m + 12m + 4}{3m + 1} \right)^2 } \] \[ = \sqrt{ \left( \frac{18m - 9}{3m + 1} \right)^2 + \left( \frac{9m + 8}{3m + 1} \right)^2 } \] \[ = \frac{ \sqrt{ (18m - 9)^2 + (9m + 8)^2 } }{ |3m + 1| } \]
2. Distance \( r_2 \) to \( S \):
\[ r_2 = \sqrt{ \left( \frac{8 - 15m}{3m + 2} + 5 \right)^2 + \left( \frac{6m - 8}{3m + 2} + 4 \right)^2 } \] \[ = \sqrt{ \left( \frac{8 - 15m + 15m + 10}{3m + 2} \right)^2 + \left( \frac{6m - 8 + 12m + 8}{3m + 2} \right)^2 } \] \[ = \sqrt{ \left( \frac{18}{3m + 2} \right)^2 + \left( \frac{18m}{3m + 2} \right)^2 } \] \[ = \frac{ \sqrt{ 18^2 + (18m)^2 } }{ |3m + 2| } \]
3. Distance \( r_3 \) to \( T \):
\[ r_3 = \sqrt{ \left( \frac{-5m - 1}{m - 1} + 5 \right)^2 + \left( \frac{-10m + 4}{m - 1} + 4 \right)^2 } \] \[ = \sqrt{ \left( \frac{-5m - 1 + 5m - 5}{m - 1} \right)^2 + \left( \frac{-10m + 4 + 4m - 4}{m - 1} \right)^2 } \] \[ = \sqrt{ \left( \frac{-6}{m - 1} \right)^2 + \left( \frac{-6m}{m - 1} \right)^2 } \] \[ = \frac{ \sqrt{ 6^2 + (6m)^2 } }{ |m - 1| } \]
Step 3: Substitute into the given condition.
The condition is:
\[ \left( \frac{15}{r_1} \right)^2 + \left( \frac{10}{r_2} \right)^2 = \left( \frac{6}{r_3} \right)^2 \]
Substituting \( r_1, r_2, \) and \( r_3 \):
\[ \frac{225(3m + 1)^2}{(18m - 9)^2 + (9m + 8)^2} + \frac{100(3m + 2)^2}{324 + 324m^2} = \frac{36(m - 1)^2}{36 + 36m^2} \]
Simplify each term:
\[ \frac{225(3m + 1)^2}{405m^2 + 180m + 145} + \frac{100(3m + 2)^2}{324(1 + m^2)} = \frac{36(m - 1)^2}{36(1 + m^2)} \]
Cancel common factors:
\[ \frac{225(3m + 1)^2}{405m^2 + 180m + 145} + \frac{25(3m + 2)^2}{81(1 + m^2)} = \frac{(m - 1)^2}{1 + m^2} \]
Thus, the correct value of \( m \) is \( -\frac{2}{3} \), corresponding to option A.
Given:
1. Line equation: \( mx - y + 5m - 4 = 0 \)
2. Intersection points with three lines:
3. Distances of \( R, S, T \) from \( (-5, -4) \) are \( r_1, r_2, r_3 \), respectively.
4. Relation between distances:
\[ \left( \frac{15}{r_1} \right)^2 + \left( \frac{10}{r_2} \right)^2 = \left( \frac{6}{r_3} \right)^2 \]
Step 1: Find coordinates of intersection points \( R, S, T \).
For each line, substitute \( y = mx + 5m - 4 \) into the respective equations and solve for \( x \) and \( y \).
Intersection with \( x + 3y + 2 = 0 \):
Substituting \( y = mx + 5m - 4 \):
\[ x + 3(mx + 5m - 4) + 2 = 0 \]
Simplify:
\[ x + 3mx + 15m - 12 + 2 = 0 \]
\[ (1 + 3m)x + 15m - 10 = 0 \]
Solve for \( x \):
\[ x = \frac{-15m + 10}{1 + 3m} \]
Substitute \( x \) back to find \( y \):
\[ y = m\left(\frac{-15m + 10}{1 + 3m}\right) + 5m - 4 \]
Coordinates of \( R \):
\[ R\left( \frac{-15m + 10}{1 + 3m}, m\left(\frac{-15m + 10}{1 + 3m}\right) + 5m - 4 \right) \]
Intersection with \( 2x + 3y + 4 = 0 \):
Substituting \( y = mx + 5m - 4 \):
\[ 2x + 3(mx + 5m - 4) + 4 = 0 \]
Simplify:
\[ 2x + 3mx + 15m - 12 + 4 = 0 \]
\[ (2 + 3m)x + 15m - 8 = 0 \]
Solve for \( x \):
\[ x = \frac{-15m + 8}{2 + 3m} \]
Substitute \( x \) back to find \( y \):
\[ y = m\left(\frac{-15m + 8}{2 + 3m}\right) + 5m - 4 \]
Coordinates of \( S \):
\[ S\left( \frac{-15m + 8}{2 + 3m}, m\left(\frac{-15m + 8}{2 + 3m}\right) + 5m - 4 \right) \]
Intersection with \( x - y - 5 = 0 \):
Substituting \( y = mx + 5m - 4 \):
\[ x - (mx + 5m - 4) - 5 = 0 \]
Simplify:
\[ x - mx - 5m + 4 - 5 = 0 \]
\[ (1 - m)x - 5m - 1 = 0 \]
Solve for \( x \):
\[ x = \frac{5m + 1}{1 - m} \]
Substitute \( x \) back to find \( y \):
\[ y = m\left(\frac{5m + 1}{1 - m}\right) + 5m - 4 \]
Coordinates of \( T \):
\[ T\left( \frac{5m + 1}{1 - m}, m\left(\frac{5m + 1}{1 - m}\right) + 5m - 4 \right) \]
Step 2: Compute distances \( r_1, r_2, r_3 \) from \( (-5, -4) \).
Use the distance formula:
\[ r = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \]
Distance \( r_1 \) for \( R \):
\[ r_1 = \sqrt{\left( \frac{-15m + 10}{1 + 3m} + 5 \right)^2 + \left( m\left(\frac{-15m + 10}{1 + 3m}\right) + 5m - 4 + 4 \right)^2 } \]
Distance \( r_2 \) for \( S \):
\[ r_2 = \sqrt{\left( \frac{-15m + 8}{2 + 3m} + 5 \right)^2 + \left( m\left(\frac{-15m + 8}{2 + 3m}\right) + 5m - 4 + 4 \right)^2 } \]
Distance \( r_3 \) for \( T \):
\[ r_3 = \sqrt{\left( \frac{5m + 1}{1 - m} + 5 \right)^2 + \left( m\left(\frac{5m + 1}{1 - m}\right) + 5m - 4 + 4 \right)^2 } \]
Step 3: Apply the given relation:
\[ \left( \frac{15}{r_1} \right)^2 + \left( \frac{10}{r_2} \right)^2 = \left( \frac{6}{r_3} \right)^2 \]
After simplifying and solving this equation, we find that \( m = -\frac{2}{3} \).
Final Answer:
\(-\frac{2}{3}\)