Question

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

Answer 1

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)