How do you find intercepts, extrema, points of inflections, asymptotes and graph f(x)=(2x^2-5x+5)/(x-2)?

1 Answer
Sep 20, 2017

Two asymptotes; two x-intercepts; one y-intercept; and two stationary points.

Explanation:

First of all do polynomial division to get your function in the nice form of:
f(x) = 2x + 1 + 3/(x-2).

From this it is fairly clear where the asymptotes are, as x cannot equal 2 as f(x) would then be undefined. Similarly f(x) cannot equal 2x + 1, as this would mean that 3/(x-2) equals 0, which it cannot as it implies 3=0.

Then differentiate this function, applying the chain rule and product rules where necessary.

The derivative of 2x + 1 is of course 2.

The derivative of 3/(x-2) is done by letting y=3(x-2)^-1.

First apply the chain rule: say t = x-2 and w=t^-1.
:. (dt)/(dx) = 1 and (dw)/(dt) = -t^-2.
:. (dw)/(dx) = -t^-2 = -(x-2)^-2

The product rule isn't really necessary as it's pretty clear you will end up with:

(dy)/(dx) = -3/(x-2)^2.

:. f'(x) = 2 -3/(x-2)^2.

Letting f'(x) = 0 next.

:. 3 = 2(x-2)^2

Simplify it down to arrive at 2x^2-8x+8 = 0, which by the quadratic formula leaves us with: x=(4-sqrt(6))/2 or x=(sqrt(6)+4)/2.

Solving for f((4-sqrt(6))/2) and f((sqrt(6)+4)/2) yields 5-sqrt(6) and 2sqrt(6) + 5, which are maximum and minimum respectively.

Finally calculate your intercepts, first by letting f(x)=0

:. -3 = (2x+1)(x-2)

:. 2x^2-3x+1 =0

:. x = 1/2 or x=1.

Finally calculate f(0) which is simply 1-3/2, therefore the y-intercept is where y=-1/2.