panjang jari-jari lingkaran x² + y² - 2x + 4y - 13 =.......?

x^2 + y^2 + Ax + By + C = 0
Jadi, A = -2, B = 4, C = -13
jari2 = √(-A/2)^2+(-B/2)^2-C
jari2 = √(1)^2+(-2)^2+13
jari2 = √1+4+13
jari2 = √18 = 3√2