JEE Challenger
More from Straight Lines
MathMedMarks to AllAdv 2013 • Paper 1

Inequalities from Distance to Intersection Point of Lines

Suppose that a>b>c>0a > b > c > 0. If the distance from (1,1)(1, 1) to the point of intersection of the pair of lines ax+by+c=0ax + by + c = 0 and bx+ay+c=0bx + ay + c = 0 is strictly less than 222\sqrt{2}, then which of the following relations must hold?

Official Notice: Marks Awarded to All

This question was dropped / full marks were awarded to all candidates in the official answer key by the exam conducting body due to an ambiguity or error in the question or options.

Options

A

a+b−c>0a + b - c > 0

B

a−b+c<0a - b + c < 0

C

a−b+c>0a - b + c > 0

D

a+b−c<0a + b - c < 0

Step-by-Step Solution

To find the point of intersection of the given pair of lines, we solve the system of linear equations: ax+by+c=0— (1)ax + by + c = 0 \quad \text{--- (1)} bx+ay+c=0— (2)bx + ay + c = 0 \quad \text{--- (2)}

Subtracting equation (2) from equation (1): (a−b)x+(b−a)y=0(a - b)x + (b - a)y = 0 (a−b)(x−y)=0(a - b)(x - y) = 0

Since a>b>c>0a > b > c > 0, we have a≠ba \neq b, which implies: x=yx = y

Substituting y=xy = x into equation (1): (a+b)x+c=0  ⟹  x=y=−ca+b(a + b)x + c = 0 \implies x = y = -\frac{c}{a + b}

Thus, the point of intersection is P(−ca+b,−ca+b)P\left(-\frac{c}{a+b}, -\frac{c}{a+b}\right).

Now, the distance dd between the point (1,1)(1, 1) and the intersection point PP is given by: d=(1−(−ca+b))2+(1−(−ca+b))2d = \sqrt{\left(1 - \left(-\frac{c}{a+b}\right)\right)^2 + \left(1 - \left(-\frac{c}{a+b}\right)\right)^2} d=2(1+ca+b)2d = \sqrt{2\left(1 + \frac{c}{a+b}\right)^2}

Since a,b,c>0a, b, c > 0, the term 1+ca+b1 + \frac{c}{a+b} is strictly positive: d=2(1+ca+b)d = \sqrt{2}\left(1 + \frac{c}{a+b}\right)

It is given that the distance is strictly less than 222\sqrt{2}: 2(1+ca+b)<22\sqrt{2}\left(1 + \frac{c}{a+b}\right) < 2\sqrt{2} 1+ca+b<21 + \frac{c}{a+b} < 2 ca+b<1\frac{c}{a+b} < 1

Since a+b>0a + b > 0, multiplying both sides by a+ba + b yields: c<a+b  ⟹  a+b−c>0c < a + b \implies a + b - c > 0 Hence, relation (A) must hold.

Additionally, from the initial hypothesis a>b>c>0a > b > c > 0: a>b  ⟹  a−b>0a > b \implies a - b > 0 Since c>0c > 0, adding cc to both sides gives: a−b+c>0a - b + c > 0 Hence, relation (C) also holds.

Therefore, the correct options are A and C.