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Questions  

The locus of the centre of a circle which cuts orthogonally the circle x2+y220x+4=0 and which touches

x=2 is

a
y2=16x+4
b
x2=16y
c
x2=16y+4
d
y2=16x

detailed solution

Correct option is D

Let the circle be x2+y2+2gx+2fy+c=0It cuts the circle x2+y2−20x+4=0 orthogonally∴    2(−10g+0×f)=c+4⇒     −20g=c+4Circle (i) touches the line x = 2 i.e. x + Oy - 2 = 0∴ −g+0−212+02=g2+f2−c⇒ (g+2)2=g2+f2−c⇒4g+4=f2−cEliminating c from (ii) and (iii), we get−16g+4=f2+4⇒f2+16g=0Hence, the locus of (−g,−f) is y2−16x=0

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The locus of the point which moves in a plane so that the sum of the squares of its distances from the lines ax+by+c=0  and bxay+d=0  is r2,  is a circle of radius.


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