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Question: Answered & Verified by Expert
If the three lines $x-3 y=p, a x+2 y=q$ and $a x+y=r$ form a right-angled triangle then :
MathematicsStraight LinesJEE MainJEE Main 2013 (09 Apr Online)
Options:
  • A
    $a^2-9 a+18=0$
  • B
    $a^2-6 a-12=0$
  • C
    $a^2-6 a-18=0$
  • D
    $a^2-9 a+12=0$
Solution:
2397 Upvotes Verified Answer
The correct answer is:
$a^2-9 a+18=0$
Since three lines $x-3 y=p$, $a x+2 y=q$ and $a x+y=r$ form a right angled triangle $\therefore$ product of slopes of any two lines $=-1$ Suppose $a x+2 y=q$ and $x-3 y=p$ are $\perp$ to each other.
$$
\therefore \frac{-a}{2} \times \frac{1}{3}=-1 \Rightarrow a=6
$$
Now, consider option one by one $a=6$ satisfies only option (a)
$\therefore$ Required answer is $a^2-9 a+18=0$

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