What is the radius of a circle whose equation is x^2+y^2-10x+6y+18=0

to find the radius of x^2+y^2-10x+6y+18=0, we must complete the square:

x^2 - 10x + y^2 + 6y = - 18

Then:

x^2 - 10x + 5^2 - 5^2 + y^2 + 6y + 9 - 9 = - 18

Collecting all three constants on the right-hand side, r^2 = - 18 + 25 + 9 = 16

Then r = + sqrt (16) = + 4 (answer)