Q.

If a circle of radius 2 touches X-axis at (1,0)  then its centre may be

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a

(1,2)  or (1,2) 

b

(1,2) or (1,2) 

c

(1,2) or (1,2) 

d

(1,2)  or (2, 1) 

answer is A.

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Detailed Solution

Let x2+y2+2gx+2fy+c=0  be the circle

Since the circle touches x-axis at (1,0)  then

g2=c  and r=|f| .

2=|f|

f=±2

Since circle passes through (1,0)  then

1+2g+c=0

1+2g+g2=0  (c=g2)

(g+1)2=0

g=1

centre=(g,f)=(1,±2)

centre is (1,2)  or (1,2)

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If a circle of radius 2 touches X-axis at (1,0)  then its centre may be