Question 77239
center is at the point (1,2) ..say x1,y1
passes through the point (4,-2).. say x2, y2 
the length of the radius of the circle = distance between the points
=sqrt [(x2-x1)^2+(y2-y1)^2]
=sqrt[(4-1)^2+(-2-2)^2]
=sqrt[(3^2)+(-4)^2
=sqrt[(9)+(16)]
=sqrt(25)
=5

The radius of the circle =5 units.
okay
good day