Math Is Fun Forum

daydoe12

Member

Registered: 2012-11-18

Posts: 8

I cant figure this out PLEASE HELP

Define a bijection between the sets [-2,8] and [3,5]. Prove your function is actually a bijection.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: I cant figure this out PLEASE HELP

hi daydoe12

Welcome to the forum.

How about

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: I cant figure this out PLEASE HELP

Hi daydoe12

Last edited by anonimnystefy (2012-11-27 02:20:37)

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: I cant figure this out PLEASE HELP

Good answer, Stefy. 

I think yours is correct and I have misunderstood what is wanted.

I'll leave the post as a reminder to me to read the question!

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: I cant figure this out PLEASE HELP

Hmmm... I thought there was another reason.

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: I cant figure this out PLEASE HELP

Which was ... ?

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: I cant figure this out PLEASE HELP

Doesn't matter. I hope we solved the OP's problem.

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

daydoe12

Member

Registered: 2012-11-18

Posts: 8

Re: I cant figure this out PLEASE HELP

how did you arrive at that answer

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: I cant figure this out PLEASE HELP

Stefy just looked for a formula of the form y = mx + c so that when x = -2, then y = 3, and similarly when x = 8, y = 5.

My answer used a similar idea but I think his is the right way.

If you think about the straight line graph this produces, every point between -2 and 8, will map onto every point between 3 and 5

And the mapping is reversible.

To prove it, you'd have to show the mapping is 1:1 in both direcctions, and that a point inside the first interval maps to a point inside the resulting interval.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

daydoe12

Member

Registered: 2012-11-18

Posts: 8

Re: I cant figure this out PLEASE HELP

anonimnystefyLegendary Memberb how did you arrive at that answer

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: I cant figure this out PLEASE HELP

Hi daydoe12

Bob explained very well in post #9. I don't think I could any better at explaining it.

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: I cant figure this out PLEASE HELP

Thank you Stefy. 

And there was me, about to award you the "Teacher of the Month Award" 

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

daydoe12

Member

Registered: 2012-11-18

Posts: 8

Re: I cant figure this out PLEASE HELP

I have tried finding the form using the y=mx+b formula and i cant figure out how she got x+17/5

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: I cant figure this out PLEASE HELP

Hi daydoe12;

You have -2 -> 3 and 8 -> 5 so call x=-2 and y =3 in the first equation and x = 8 and y = 5 in the second.

When you solve that you get m = 1/5 and b = 17 / 5. Now plug in to the first equation.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: I cant figure this out PLEASE HELP

Thank you Stefy. 

And there was me, about to award you the "Teacher of the Month Award" 

Bob

I hope you are kidding.

Hi daydoe12

I'm male, not female...

---

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.