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Find the radius of the circle \(x^{2} + y^{2} - 8x - 2y + 1 ...

Find the radius of the circle x2+y28x2y+1=0.

A.

9

B.

7

C.

4

D.

3

Correct answer is C

Given the equation of the circle x2+y28x2y+1=0.

The equation of a circle is given as (xa)2+(yb)2=r2

Expanding, we have x22ax+a2+y22by+b2=r2x22ax+y22by=r2a2b2

Comparing the RHS of the equation above with the equation rewritten as x2+y28x2y=1, we have

2a=8;2b=2a=4,b=1

r = \sqrt{16} = 4