Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

Hey guys!

Well I've been trying to do previous exams as practice for the one I have next week.

I came across 2 questions, of which I have no idea how to solve.

If you guys could show working and explanations so that I understand as well, that'll be great.

Q 1

While attempting to solve a quadratic equation, Christobel inadvertently interchanged the coefficient of x² with the constant term, causing the equation to change.

She solved this different equation accurately.

One of the roots she got was 2 and the other was a root of the original equation.

What are the sum of the squares of the two roots of the original equation?

I tried letting the first equation be ax²+bx+c=0 and the other be cx²+bx+a=0 but then I didn't know what to do...

Q 2

A goods train leaves Southampton at 9.17 am and arrives in London at 12.02 pm.

On the same day, a passenger express train leaves Southampton at 9.56 am and arrives in London at 11.36 am.

At what time does the passenger train pass the goods train if each is travelling non-stop at constant speed?

Yeah... I didn't even bother trying this question, I have no clue as to what variables I need to make or such..

Any help is appreciated. Thanks guys.

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi;

I have a question are a,b,c integers?

**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.**

Offline

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

Yeah, probably

I used the quadratic formula to solve for the two equations,

but I don't know which one to be 2 or to be equal with the other roots from the other equation

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

Hi Denominator

You should probably not done that. Instead you should have x-2 as a factor of that second polynomial.

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.

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,541

hi Denominator,

I'll have a go at these but it'll be in about half an hour if that's OK with you.

No problem whether a, b and c are integers or not.

Back shortly.

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 Bundy

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

Hi Denominator

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.

Offline

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

Oh okay anonimnystefy,

so I make the equation: (x-2)(ax+b)=0

ax²+(b-2a)x-2b=0

And the other equation is -2bx²+(b-2a)x+a=0

Which factorises to (bx+a)(-2x+1)=0

And the solutions are -a/b and 1/2

Okay so either -b/a=-a/b or -b/a=1/2

Doesn't that mean theres still many solutions for a and b?

Also, I don't have the answers so I'm trusting you guys ;S

*Last edited by Denominator (2012-03-23 21:42:17)*

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi;

**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.**

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

What are those?

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.

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

The sums of the squares of the roots, I think.

**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.**

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

Look at mine.

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.

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,541

hi Denominator

This is the corrected version.

let the equations be

and

multiplying out

and

equating coefficients gives

from (ii) and (iii)

and putting (ii) into (i) gives

Putting (iv) into (v) gives

which looks a bit nasty. But you can easily spot that R - 1 is a factor which leaves a quadratic, which factorises easily and it should be plain sailing from there.

Bob

ps. Now to read number 2

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 Bundy

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi anonimnystefy;

Who should be looking?

**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.**

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

You, m.

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.

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

There are other solutions if we relax the integer coefficients idea. What are your quadratic two equations?

**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.**

Offline

**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,039

They are both 2x^2-5x+2.

*Last edited by anonimnystefy (2012-03-23 22:14:34)*

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.

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

That equation is illegal.

You also should have posted a new thread instead of tacking on to an active one.

**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.**

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,541

hi Denominator

trains:

For questions like this I like to sketch a distance time graph so i get a good picture in my head of what is happening. see below.

You have enough data to work out the gradients for the two lines. And you know that at the moment of passing the 't' values and the 'd' values are equal.

So you could make two equations for d/t based on the gradients and solve for d and t.

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 Bundy

Offline

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

Oh okay thanks guys!

Yeah, after using your methods, I get 5/4 as well

I tried doing something different:

Let the solutions of the different equation be X and 2

The solutions of the original equation is 1/X and 1/2 (after playing around, I found this rule)

So 1/X=X

Which means X=1 or -1

Sum of squares (1)²+(1/2)²=5/4

Btw even though 2x²-5x+2 works, the question says 2 different equations, 1 original and the other with the coefficients swapped... this stays the same.

Thanks a lot guys!

As for Q2

Can you tell me what equations you formed, because I couldn't make some..

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi;

Isn't 5 also an answer?

**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.**

Offline

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

I don't think so because the question asks for the sum of the roots of the ORIGINAL equation, not the different equation

That is if you're thinking the roots are 2 and 1, but that is from the different equation, not the original

Unless your saying the roots of the original equation are 1/2 and √19 / 2

Which I don't think is right

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Why can't you switch the equations calling the second one the original one like this?

Original equation:

Different 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.**

Offline

**Denominator****Member**- Registered: 2009-11-23
- Posts: 155

Because the question says that the different equation has to have 2 as a root

Offline

**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi;

Okay, yes you are right, I did not see that.

**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.**

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,541

hi Denominator

Let the distance from Southampton to London be D, and the distance to the overtaking point be d.

Let the time in minutes from 9.17 to the overtaking point be t

The goods train takes 165 minutes

The express train takes 100 minutes

So the gradient (velocity in miles / min) for the goods train is

and for the express train

You can eliminate both D and d by dividing these equations:

and then solve for t.

Bob

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 Bundy

Offline