# Mumsnet Talk

## Pls help. Maths question for 14 year old.

(10 Posts)
Tue 24-Sep-13 16:35:51

We're in France and all Maths vocab in French is a little alien to me. I'm trying to help my DS with his homework (there have been tears) but my memory isn't what it should be.

He has to develop: A = (2x + 1)(2x - 1)-3(x -5)squared (sorry on phone - can't do the little sq sign)

Your help would be much appreciated as there are 26 similar exercises to do.

Jenny70 Tue 24-Sep-13 16:42:20

So the brackets multiply each term out:
4×^2 (squared!) -2× + 2× -1

Which makes 4x^2 -1

Then second half multiplies 3 to both term, so becomes 3x-15

Joining becomes
4x^2 -1-3×+15 (take away neg makes +)
4×^2 -3× + 14 = A

Hope that is clear (and correct!)

KatOD Tue 24-Sep-13 16:49:41

Yep what Jenny said

Tue 24-Sep-13 16:54:26

We are busy trying to apply the (a-b)(a+b) = a^2-b^2 type formulae.

A = (2x+1)(2x-1)-3(x-5)^2
A = (2x^2-1^2)-3(x^2-10x+25)

Are we completely on the wrong track?

Thanks Jenny for your answer.

Tue 24-Sep-13 17:07:45

He needs to break it right down if he's unsure of the process:

(2x+1)(2x-1)-3(x-5)^2=

first, brackets:

2x x 2x = 4 x x^2 = 4x^2

2x x -1 = -2x

1 x 2x = 2x

1 x -1 = -1

so, 4x^2 -2x +2x -1 which = 4x^2 -1

3(x-5)^2 = 3(x-5)(x-5) = 3(x^2 -5x -5x +25) = 3x^2 -10x +25

the overall sum:

4x^2 - 3x^2 - 10x - 1 + 25 = x^2 -10x +24

Tue 24-Sep-13 17:32:35

Lougle, I'm with you up until removing the brackets on the -3 side.

I think:
-3(x^2-5x-5x+25) =
-3(x^2-10x+25) =
-3x^2+30x+25 ????

Tue 24-Sep-13 17:50:42

-3x^2+30x-75 ???

Tue 24-Sep-13 17:55:02

You're right, I forgot to multiply through by 3 sorry.

It should read:

3(x-5)^2 = 3(x-5)(x-5) = 3(x^2 -5x -5x +25) = 3x^2 -30x +75

the overall sum:

4x^2 - 3x^2 - 30x - 1 + 75 = x^2 -30x +74

because you must multiply all the terms.

If you stop where you stopped in your post just now, you've only multiplied the first term by 3, instead of multiplying the contents of the whole bracket.

Jenny70 Tue 24-Sep-13 18:02:30

Sorry, I forgot the squared on the second term!

Tue 24-Sep-13 18:03:18

Hey hey success. What you say is on my bit of scrap paper. I shall try to explain as best I can to DS who would prefer that I say it all in French...

Thanks for your help .

### Join the discussion

Registering is free, easy, and means you can join in the discussion, get discounts, win prizes and lots more.

Register now