x^2-30x+225 = 0

D - b^2 - 4ac = (-30)^2 - 4*1*225 = 900 - 900 = 0

D = 0 - 1 корень уравнения

x = b/2 = 30/2 = 15

Verified answer

x^2-30x+225=(x-15)^2

x-15=0

x=15