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Question

Let 2x2+y23xy=0 be the equation of a pair of tangents drawn from the origin O to a circle of radius 3 units with centre in the first quadrant. If A is one of the points of contact, then the length of OA is

A
3+10 units
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B
9(1+10) units
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C
3(3+10) units
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D
9+10 units
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Solution

The correct option is C 3(3+10) units
y23xy+2x2=0
Let θ be the angle between the two lines. Then,
tanθ=2(9/4)21+2=13
but from angle between the formulae
tanθ=2LrL2r2,
Hence, 13=6LL29
L218L9=0
L=9±90
As L can't be negative,
L=9+90
L=3(3+10)
OA=L=3(3+10) units

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