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Question

If the line xa+yb=1 intersects the curve 5x2+5y2+5bx+5ay9ab=0 at P and Q such that POQ=90, where O is the origin, then the value of ab is

A
12
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B
2
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C
13
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D
3
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Solution

The correct option is B 2
xa+yb=1 (1)
5x2+5y2+5(bx+ay)19ab(1)2=0 (2)
Combined equation of lines OP and OQ can be obtained by homogenization of (2) w.r.t. to (1)
5x2+5y2+5(bx+ay)(xa+yb)9ab(xa+yb)2=0
(54ba)x2+(54ab)y28xy=0
Since, lines are perpendicular,
10=4(ab+ba)
ab+ba=52

Let ab=t
t+1t=52
2t25t+2=0
(2t1)(t2)=0
t=ab=12 or 2

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