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Question

The equations of circles with radius 3 units and touching the circle x2+y22x4y20=0 at (5,5) is/are

A
(5x16)2+(5y13)2=225
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B
(5x13)2+(5y16)2=225
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C
(5x34)2+(5y37)2=225
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D
(5x37)2+(5y34)2=225
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Solution

The correct option is D (5x37)2+(5y34)2=225

Given circle is x2+y22x4y20=0
C=(1,2),r=5

As seen from figure, there are 2 possible circles of 3 units,
Now, slope of the line joining centres
tanθ=5251=34cosθ=45,sinθ=35

Using parametric form of line
The coordinates of C1 and C2 are
=(5±3cosθ,5±sinθ)=(5±3(45),5±3(35))C1=(135,165), C2=(375,345)

The required equations of the circles are
(x135)+(y165)=9(5x13)2+(5y16)2=225(x375)+(y345)=9(5x37)2+(5y34)2=225




Alternate solution:
Given circle is x2+y22x4y20=0
C=(1,2),r=5
Let the centre of the circle be (h,k)
Now,
k5h5=5251k5h5=34(1)
And
(h5)2+(k5)2=32
Using equation (1), we get
25(h5)216=9h5=±125h=135,375k=165,345

So, the equation of circles are
(x135)+(y165)=9(5x13)2+(5y16)2=225(x375)+(y345)=9(5x37)2+(5y34)2=225

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