Pls help. Maths question for 14 year old.

[zimbomaman]

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]

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]

Yep what Jenny said

[zimbomaman]

We are busy trying to apply the (a-b)(a+b) = a2-b2 type formulae.

A = (2x+1)(2x-1)-3(x-5)^2
A = (2x2-12)-3(x^2-10x+25)

Are we completely on the wrong track?

Thanks Jenny for your answer.

[lougle]

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 x2 = 4x2

2x x -1 = -2x

1 x 2x = 2x

1 x -1 = -1

so, 4x2 -2x +2x -1 which = 4x2 -1

3(x-5)2 = 3(x-5)(x-5) = 3(x2 -5x -5x +25) = 3x^2 -10x +25

the overall sum:

4x2 - 3x2 - 10x - 1 + 25 = x^2 -10x +24

[zimbomaman]

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 ????

[zimbomaman]

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

[lougle]

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

It should read:

3(x-5)2 = 3(x-5)(x-5) = 3(x2 -5x -5x +25) = 3x^2 -30x +75

the overall sum:

4x2 - 3x2 - 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]

Sorry, I forgot the squared on the second term!

[zimbomaman]

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 .