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Straight Lines and Pair of Straight Lines question

2014 · Shift 0 · Q37
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  5. /2014 · Shift 0 · Q37

Straight Lines and Pair of Straight Lines question

2014 · Shift 0 · Q37

JEE MainMathematicsStraight Lines and Pair of Straight LinesMCQ+4 / −1
Let a,b,ca, b, ca,b,c and ddd be non-zero numbers. If the point of intersection of the lines 4ax+2ay+c=04ax + 2ay + c = 04ax+2ay+c=0 and 5bx+2by+d=05bx + 2by + d = 05bx+2by+d=0 lies in the fourth quadrant and is equidistant from the two axes then :
  1. A
    3bc−2ad=03bc - 2ad = 03bc−2ad=0
  2. B
    3bc+2ad=03bc + 2ad = 03bc+2ad=0
  3. C
    2bc−3ad=02bc - 3ad = 02bc−3ad=0
  4. D
    2bc+3ad=02bc + 3ad = 02bc+3ad=0
View written solutionFree

Correct answer: A

  1. Given lines

The two lines are: 4ax+2ay+c=04ax+2ay+c=04ax+2ay+c=0 5bx+2by+d=05bx+2by+d=05bx+2by+d=0

We need the point of intersection of these lines.


  1. Find the intersection point

From 4ax+2ay+c=04ax+2ay+c=04ax+2ay+c=0 we get 2a(2x+y)=−c2a(2x+y)=-c2a(2x+y)=−c 2x+y=−c2a2x+y=-\frac{c}{2a}2x+y=−2ac​

From 5bx+2by+d=05bx+2by+d=05bx+2by+d=0 we get b(5x+2y)=−db(5x+2y)=-db(5x+2y)=−d 5x+2y=−db5x+2y=-\frac{d}{b}5x+2y=−bd​

So we solve the system: 2x+y=−c2a...(1)2x+y=-\frac{c}{2a} \quad ...(1)2x+y=−2ac​...(1) 5x+2y=−db...(2)5x+2y=-\frac{d}{b} \quad ...(2)5x+2y=−bd​...(2)

Multiply (1) by 2: 4x+2y=−ca4x+2y=-\frac{c}{a}4x+2y=−ac​

Subtract this from (2): (5x+2y)−(4x+2y)=−db+ca (5x+2y)-(4x+2y)=-\frac{d}{b}+\frac{c}{a}(5x+2y)−(4x+2y)=−bd​+ac​ x=ca−dbx=\frac{c}{a}-\frac{d}{b}x=ac​−bd​

Taking LCM: x=bc−adabx=\frac{bc-ad}{ab}x=abbc−ad​

Now from (1): y=−c2a−2xy=-\frac{c}{2a}-2xy=−2ac​−2x y=−c2a−2(bc−adab)y=-\frac{c}{2a}-2\left(\frac{bc-ad}{ab}\right)y=−2ac​−2(abbc−ad​)

Using common denominator 2ab2ab2ab: y=−bc−4bc+4ad2aby=\frac{-bc-4bc+4ad}{2ab}y=2ab−bc−4bc+4ad​ y=4ad−5bc2aby=\frac{4ad-5bc}{2ab}y=2ab4ad−5bc​

Thus intersection point is (bc−adab,  4ad−5bc2ab)\left(\frac{bc-ad}{ab},\; \frac{4ad-5bc}{2ab}\right)(abbc−ad​,2ab4ad−5bc​)


  1. Use the condition: point lies in fourth quadrant and is equidistant from axes

A point equidistant from the two axes satisfies ∣x∣=∣y∣|x|=|y|∣x∣=∣y∣

Since the point is in the fourth quadrant, we have x>0,y<0x>0,\quad y<0x>0,y<0 Therefore, y=−xy=-xy=−x

So, 4ad−5bc2ab=−bc−adab\frac{4ad-5bc}{2ab}=-\frac{bc-ad}{ab}2ab4ad−5bc​=−abbc−ad​

Multiply both sides by 2ab2ab2ab: 4ad−5bc=−2(bc−ad)4ad-5bc=-2(bc-ad)4ad−5bc=−2(bc−ad) 4ad−5bc=−2bc+2ad4ad-5bc=-2bc+2ad4ad−5bc=−2bc+2ad

Bring all terms to one side: 4ad−5bc−2ad+2bc=04ad-5bc-2ad+2bc=04ad−5bc−2ad+2bc=0 2ad−3bc=02ad-3bc=02ad−3bc=0

Hence, 3bc−2ad=03bc-2ad=03bc−2ad=0


  1. Match with options

This is exactly: 3bc−2ad=0\boxed{3bc-2ad=0}3bc−2ad=0​ So the correct option is:

A


  1. Comparison with stored answer

Stored correct answer: A

Our derived answer: A

They agree.

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