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how do you work out 4(a+1)(a-8)

lydiafoof

please explain

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Reply 1

ombtom

Original post by lydiafoof

please explain

Assuming 4(a+1)(a-8) = 0:

Each factor could = 0, as anything multiplied by 0 = 0.

4 cannot equal 0; either (a+1) or (a-8) equals 0.

a + 1 = 0 --> a = -1.

a - 8 = 0 --> a = 8.

Reply 2

sandvika

you do (4a+4)(a-8)

4a = -4

a=-1

a = 8

(edited 8 years ago)

Reply 3

Kevin De Bruyne

Original post by lydiafoof

please explain

It's not equal to anything. Do you mean expand?

If so, what do you think you should do/what have you been taught?

Reply 4

Zacken

Original post by lydiafoof

please explain

I assume you want to simplify it?

You can do it like this:

4(a+1)(a-8) = 4 \times \bigg[(a+1)(a-8)\bigg] = 4 \times \bigg[a(a-8) +1(a-8)\bigg] = 4\times \bigg[a \times a - a \times 8 + 1 \times a - 1 \times 8\bigg]

Can you take it from here?

(edited 8 years ago)

Reply 5

OrionMusicNet

Original post by lydiafoof

please explain

Ignore the 4 at first and expand normally. so (a+1)(a-8) expanded is = a^2 - 7a - 8. Then multiply it all by 4 so 4(a^2 - 7a -8) which = 4a^2 - 28a - 32. If I am wrong then I am going to edit this comment and delete the contents.

(edited 8 years ago)

Reply 6

sandvika

Original post by OrionMusicNet

thats not solving but you dont want to multiply by 4 last

Reply 7

Zacken

Original post by OrionMusicNet

Original post by sandvika

thats not solving but you dont want to multiply by 4 last

Why doesn't he want to multiply by 4? The question hasn't mentioned anything about solving, I don't know where you're getting that from.

The one thing that doesn't quite sit with me is that he's posted a full solution, which is against forum guidelines. :smile:

Reply 8

jamestg

Original post by ombtom

Assuming 4(a+1)(a-8) = 0:

Each factor could = 0, as anything multiplied by 0 = 0.

4 cannot equal 0; either (a+1) or (a-8) equals 0.

a + 1 = 0 --> a = -1.

a - 8 = 0 --> a = 8.

Did a better job explaining that than my Y8 maths teacher lmao. Ahhh all those years ago.

Reply 9

OrionMusicNet

Original post by Zacken

I didn't even know it was a forum guideline. Will take better care next time, my bad.

Reply 10

ombtom

Original post by jamestg

Did a better job explaining that than my Y8 maths teacher lmao. Ahhh all those years ago.

Yeah I got fed up with school maths teachers so I taught myself. :wink:

Reply 11

jamestg

Original post by ombtom

Yeah I got fed up with school maths teachers so I taught myself. :wink:

Literally the same, went from a 5A to 7A in one term (apparently that's two years of progress) because I decided to give up hope in my teachers and used YouTube instead.

Reply 12

Zacken

Original post by OrionMusicNet

I didn't even know it was a forum guideline. Will take better care next time, my bad.

It's fine, as long as you're aware of it from now on! :smile:

Reply 13

sandvika

Original post by Zacken

I said he doesn't want to multiply by 4 LAST, he will want to do it FIRST. And in what world does "work out" not mean "solve"???? And where does it say to convert it into quadratic form?

Reply 14

georgia7600

Original post by lydiafoof

please explain

I'm assuming you mean expand and simplify, if so you need to use FOIL.

This is a good technique for expanding as it gives you the answer in some quick steps. I will explain the method below :smile:

.

FOIL-

4(a+1)(a-8) means you need to times the first brackets by 4.

Keep the numbers in the brackets ready to expand and simplify.

This will give you: (4a+4)(a-8)

You need to now times everything together (but in a certain way).

First
Outer
Inner
Last

Go in order so you multiply the FIRST numbers together:
4a x a = 4a²

^{Now the OUTER numbers:}
4a x -8 = -32a

So far we have 4a²- 32a

Next multiply the INNER numbers, so:
4 x a = 4a

Finally the LAST numbers:
4 x -8 = -32

If we put it all together we get: 4a²- 32a + 4a - 32

We now need to simplify this so we just put together the numbers that match.

4a² is the only squared number so we leave this as it is.

However, we have two lots of numbers that have 'a' after so we do -32a + 4a = -28a

32 is also the only alone number so we leave this as well.

Put this together and it equals: 4a²- 28a -32

Simple when you know how :smile:

Don't hesitate to ask about anything else :u:

Reply 15

ombtom

Original post by jamestg

Literally the same, went from a 5A to 7A in one term (apparently that's two years of progress) because I decided to give up hope in my teachers and used YouTube instead.

They said I couldn't do A-level maths so I used YouTube and got a better grade than a lot of their students. :mwuaha:

Reply 16

Zacken

Original post by sandvika

I said he doesn't want to multiply by 4 LAST, he will want to do it FIRST. And in what world does "work out" not mean "solve"???? And where does it say to convert it into quadratic form?

Nopes, he wants to do it last and that's perfectly fine. I would find it easier and faster to multiply it last as well, as would a computer as would you in almost any other mathematical setting like matrices and scalars.

Work out could mean simplify, I'm not pretending to understand what the OP means, I'm simply saying that you cannot determine what the OP means either, neither of us know, so don't criticise other people for having different interpretations for the OP's vague post that the OP needs to rectify him/herself.

Just because you think the OP means solve, doesn't mean that the OP actually means solve.