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Question

For a>b>c>0, the distance between (1,1) and the point of intersection of the lines ax+by+c=0 and bx+ay+c=0 is less than 22. Then

A
a+bc>0
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B
ab+c<0
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C
ab+c>0
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D
a+bc<0
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Solution

The correct option is C ab+c>0
ax+by+c=0 (1)
bx+ay+c=0 (2)

(1)×a(2)×b, we get
x=ca+b
Since, the two lines are symmetric about the line y=x, the intersection point lies on the line y=x.
y=ca+b
Or, substitute the value of x in eqn (1), we get y=ca+b

Now, (1+ca+b)2+(1+ca+b)2<22

Squaring both the sides (as terms are positve)

2(a+b+ca+b)2<8

(a+b+ca+b)2<4

2<(a+b+ca+b)<2

2a2b<a+b+c<2a+2b

2a2b<a+b+c or a+b+c<2a+2b

3a3bc<0 or ab+c<0

3a+3b+c>0 or a+bc>0

Since, a+bc>0 and a>b
ab+c>0

Hence, both a+bc>0 and ab+c>0 are correct.

Alternate solution:
a>b>c>0
ab>0 and ac>0
ab+c>0 and a+bc>0

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