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Previous Year Question (PYQs)
2
Let a triangle be bounded by the lines L1 : 2x + 5y = 10; L2 : $-$4x + 3y = 12 and the line L3, which passes through the point P(2, 3), intersects L2 at A and L1 at B. If the point P divides the line-segment AB, internally in the ratio 1 : 3, then the area of the triangle is equal to :
Solution
Let $A=(a,b)\in L_2\Rightarrow -4a+3b=12.$
Since $AP:PB=1:3$, by section formula $P=\dfrac{B+3A}{4}\Rightarrow B=4P-3A=(8-3a,\;12-3b).$
Because $B\in L_1$, $2(8-3a)+5(12-3b)=10\Rightarrow 2a+5b=22.$
Solve
\[
\begin{cases}
2a+5b=22,\\
-4a+3b=12
\end{cases}
\Rightarrow a=\dfrac{3}{13},\quad b=\dfrac{56}{13}.
\]
Thus
\[
A=\left(\dfrac{3}{13},\dfrac{56}{13}\right),\quad
B=\left(\dfrac{95}{13},-\dfrac{12}{13}\right).
\]
Intersection $C=L_1\cap L_2$:
\[
\begin{cases}
2x+5y=10,\\
-4x+3y=12
\end{cases}
\Rightarrow C=\left(-\dfrac{15}{13},\dfrac{32}{13}\right).
\]
Area
\[
\Delta=\frac12\left|
\begin{vmatrix}
x_A&y_A&1\\
x_B&y_B&1\\
x_C&y_C&1
\end{vmatrix}\right|
=\frac12\left|(B-A)\times(C-A)\right|
=\frac12\left|(92)(-24)-(-68)(-18)\right|
=\frac{1716}{169}
=\boxed{\dfrac{132}{13}}.
\]
Answer: $\boxed{\dfrac{132}{13}}$.
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