How do you sketch the graph of f(x) = (x^2 - 3x + 9)/(x - 3)?

1 Answer
Apr 4, 2015

f(x) = (x^2 - 3x + 9)/(x - 3) = (x^2 - 3x)/(x - 3) + 9/(x - 3)

= (x(x - 3))/(x - 3) + 9/(x - 3) = x + 9/(x - 3)

So you actually wanna sketch f(x) = x + 9/(x - 3)

Domain:
The domain of this function is all Real x except 3
because 9/(x - 3) would become unreal!

This brings us to Asymptotes:
They're two Asymptotes:

No 1: y = x because as x -> oo f(x) -> x

No 2: x= 3 is a vertical asymptote because as f(x) -> oo f(x)-> 3

Next, turning points:
To find turning points (maxima and minima),
We let y = f(x) => y = x + 9/(x - 3)

We make x the subject of the equation,
=> yx - 3y = x^2 - 3x + 9
=> x^2 -(3 + y)x + 9+ 3y = 0

Since all the values of x are real, b^2 - 4ac >= 0

=> (3 + y)^2 - 4(9 + 3y) >= 0

=> y^2 - 6y - 27 >=0 => (y - 9)(y + 3)>= 0

Hence, y<=-3 uu y >= 9

Putting the values of y in f(x) to get x
we have the points (0, -3) maximum and (6, 9) as minimum

To find the intercepts :
y- intercept is when x = 0 we have (0, -3)
x- intercept is when y = 0 but y cannot be 0(see the range above)

Table of variation:
enter image source here

So now we're all set to sketch that graph;
Here's what i got:
enter image source here