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From Förberedande kurs i matematik 1

If the circle is to contain the point −11 , then that point's distance away from the centre 2−1  must equal the circle's radius, r. Thus, we can obtain the circle's radius by calculating the distance between −11  and 2−1  using the distance formula:

r=2−−12+−1−12=32+−22=9+4=13 

When we know the circle's centre and its radius, we can write the equation of the circle,

x−22+y−−12=132 

which the same as

x−22+y+12=13 

NOTE: A circle having its centre at ab  and radius r has the equation

x−a2+y−b2=r2