1. ## multiplication

hi, i need to expand these brackets , i know the answer will be 20 but i need to know what =

2. Originally Posted by andyboy179
hi, i need to expand these brackets , i know the answer will be 20 but i need to know what =
$(2-4\sqrt{3})(2+4\sqrt{3}) \Rightarrow 2^2 - (4\sqrt{3})^2 \Rightarrow 4-48 = -44$

$-2\times4\sqrt{3} = -8\sqrt{3}$

Edit:apparently I didn't go down the right path with this, as per definition of "expansion"

3. at school we are told to work it out like this, (the blank spaces are the ones i don't know)

4. RULE: (x + y)(x - y) = x^2 - y^2

By the way, that's not "expanding brackets": it's a multiplication.

5. Originally Posted by andyboy179
at school we are told to work it out like this, (the blank spaces are the ones i don't know)
BUT 4sqrt(3) times 4sqrt(3) = 48, not 24

6. Originally Posted by Wilmer
BUT 4sqrt(3) times 4sqrt(3) = 48, not 24
oh ye, i know what i did wrong! so what would -2 x 4sqrt(3)= ?

7. Originally Posted by andyboy179
oh ye, i know what i did wrong! so what would -2 x 4sqrt(3)= ?
Doesn't matter, because you also need to do +2 x 4sqrt(3), so the
2 will result in zero.

8. Originally Posted by andyboy179
at school we are told to work it out like this, (the blank spaces are the ones i don't know)
The method that you use in school, although correct, is not the method used by most (and when I say most-I mean almost all) institutions. A really good explaination of how these problems are done can be found here.

algebra.help -- Simplifying using the FOIL Method

BTW: Your method is not an "expansion" in the normal sense of the word, as someone has already mentioned. THe term expansion implies that there will be more terms after the mutiplication is complete (in most cases). And that thes terms may have likenesses.