# Polynomial Functions.

mtayab1994

f:]1,+∞[→]2,+∞[
x→ 2x/(x-1)

## Homework Equations

Prove that f is injective and serjective.

## The Attempt at a Solution

I already proved that it's injective by stating the injectivity law:
for every (a,b)ε]1,+∞[: f(a)=f(b) implies a=b

so: 2a/(a-1)=2b/(b-1) entails: 2ab-2b=2ab-2a entails -2b=-2a entails a=b

Can anyone please tell me how to prove that its serjective?

Homework Helper
Gold Member
Sure. If y is in ]2,+∞[, calculate what x in ]1,+∞[ gives f(x) = y.

Homework Helper
Dearly Missed

f:]1,+∞[→]2,+∞[
x→ 2x/(x-1)

## Homework Equations

Prove that f is injective and serjective.

## The Attempt at a Solution

I already proved that it's injective by stating the injectivity law:
for every (a,b)ε]1,+∞[: f(a)=f(b) implies a=b

so: 2a/(a-1)=2b/(b-1) entails: 2ab-2b=2ab-2a entails -2b=-2a entails a=b

Can anyone please tell me how to prove that its serjective?

In your own words, what does it mean to say that f is surjective? (That is sUrjective, not sErjective!) Turn that verbal statement into an equation and then work on the equation, to see what conclusions you can make, or else use some known, general properties to get a conclusion.

RGV

mtayab1994
i got it
f(x)=y
y=2x/x-1 equivalence y(x-1)=2x equivalence yx-2x-y=0
now we find Δ
Δ=4+4y^2
since Δ≥0 therefore there is some solution to this equation and therefore f is serjective.

Homework Helper
Dearly Missed
What does $\Delta = 4 + 4y^2$ have to do with anything here? Anyway, you are still spelling surjective incorrectly.

RGV

mtayab1994
What does $\Delta = 4 + 4y^2$ have to do with anything here? Anyway, you are still spelling surjective incorrectly.

RGV

Well my teacher stated that if we find that Δ≥0 then therefore f is surjective and btw my first language is english, but i'm learning overseas in Morocco and all the lessons here are in Arabic, so that's probably the reason why i spelled it wrong.

Gold Member
MHB
What does $\Delta = 4 + 4y^2$ have to do with anything here? Anyway, you are still spelling surjective incorrectly.

RGV

i believe he's taking the discriminant of a quadratic.