Can someone explain this to me? Please.

Discussion in 'Community Discussion' started by furious, Feb 27, 2007.

1. furious macrumors 65816

Joined:
Aug 7, 2006
Location:
Australia
#1
How does 5 * (2 *a - 6*b^2) become 10 * a - 30 * b^2?

I am doing revision as I have my first class of Quantitative Analysis for Business. All the maths I am fine with but Distributive Property of Multiplication. This is really making me nervous because getting a tutor through the Uni is impossible. I think it is something really simple if I could just find how this problem is solved. Thank you if you can help.

ps Does anyone know of any web sites that might be of help. I have not looked yet.

Joined:
Jan 8, 2003
Location:
Georgia
#2
I'm confused.....are you having trouble seeing that 5*2 is 10 and that 5*6 is 30....therefore 10a-30b^2? Is that what you are asking? You are just distributing the 5 through to the other two (2a & 6b^2).

Staff Member

Joined:
Jan 20, 2005
#3
Have a look at this page to see if that helps explain things.

When you have a sum (or a difference) in parentheses that is multiplied by something outside of the parentheses, the distributive property says that you can multiply each of the individual terms inside the parentheses by the multiplier.

2(x+y) = 2x+2y
2(x-y) = 2x-2y

Joined:
Jan 8, 2003
Location:
Georgia
#4
I wish my test was on this tomorrow and not on linearization, differentials, and derivatives.....

5. gauchogolfer macrumors 603

Joined:
Jan 28, 2005
Location:
American Riviera
#5
Edit to avoid confusion: the stuff in bold should be 2a & 6b^2

Everything else you wrote is fine.

Joined:
Jan 8, 2003
Location:
Georgia
#6
oops....and that is where the silly mistakes come in.....-5 for me.

Thanks.

Joined:
Oct 11, 2004
Location:
On the Left Coast - Victoria BC Canada
#7
Because the outside term has to be appied to each of the inside terms as separated by + or - operators

5(a+b) = 5*a + 5*b

Now, it doesn't matter particularly what the 5, the a and b are, as long as you apply the outside term to all of the insides.

5*(2*a - 6*b^2) =
(5*2*a) - (5*6*b^2) =
(10*a) - (30*b^2)

The tricky bit, which many people stumble on, is that if the outside term is a negative number, you have to carry through on the negative sign all the way through.

-5 (a + b - c) =
(-5*a) + (-5*b) - (-5*c) =
-5a - 5b + 5c

(as the two -'s make it +)

8. furious thread starter macrumors 65816

Joined:
Aug 7, 2006
Location:
Australia
#8
Doh! I knew it would be so stupid as to make me blind. I have been on holidays for the last 4 months. I think I need a break.

Wow! Can you be my teacher please. The Guy is a nut. I had him for Economics last year.

Thanks for the web site. Looks to be really helpful. Very easy to understand the explanations.