Domena is a set of arguments , the value x.
We need to reject SUCH values of x , which must not be used . We know that you can not divide by zero , so the denominator can not equal zero.
We compute what value of x will give us the denominator 0
x² - 9x + 18 = 0
a = 1 , b= -9 , c=18 - coefficients
discriminant of a quadratic equation
Δ = b² - 4ac = (-9)² -4 1 18 = 81 - 72 = 9 ⇔ √Δ = 3
zeros of functions
\(x_{1} = \frac{-b - \sqrt{delty} }{2*a} = \frac{-(-9) - 3}{2* 1} = \frac{9 -3}{2} = 3 \\ \\ x_{2} = \frac{-b+ \sqrt{delty} }{2*a} = \frac{-(-9) + 3 }{2*1} = \frac{12}{2} = 6\)
D = R/ {3,6}
