MetaChat - Math is hard.

MetaChat is an informal place for MeFites to touch base and post, discuss and chatter about topics that may not belong on MetaFilter. Questions? Check the FAQ. Please note: This is important.

21 November 2007

Math is hard. I spent part of yesterday evening helping the g/f's daughter with her math homework. →[More:] She had to calculate the width and height of a rectangle from the perimeter and area. Firstly, is it just me or is that an insanely difficult task to give to an 11 year old. I'm pretty sure that we never did anything like that until I was 13 or 14.

Secondly... other than splitting the area down into denominators and adding them up again to see if you get the desired perimeter (the approach we took), how the hell would you even do this. My rusty algebra wasn't up to the task.

post by: seanyboy at: 02:42 | 10 comments

You know the area a, and the perimeter p. Call the two sides x and y.

Perimeter:
p = x + x + y + y
p = 2(x+y)

Area:
a = xy
x = a/y

Substitute x=a/y into perimeter equation:
2(a/y + y) = p

Let a = 20 and p = 18:
2(20/y + y) = 18
20/y + y = 9
20 + y² = 9y

Factorise y² - 9y + 20 = 0:
(y - 5)(y - 4) = 0

y = 5 or y = 4.

If y = 5, x = 4 (since xy = 20) and vice versa. So the rectangle is 5 by 4.

posted by matthewr 21 November | 04:02

Metachat stole all the plus signs in my comment! HTML entities (like &43;) don't seem to work either. On the bright side, it's clear where the pluses should be, since there's a space where they have been removed, and I left no spaces where I meant multiply.

posted by matthewr 21 November | 04:10

it's clear where the pluses should be, since there's a space where they have been removed, and I left no spaces where I meant multiply.

Uh, yeah ... of course.

*head asplodes*

posted by essexjan 21 November | 04:28

God Damn It.
I think my overzealous field cleaning (urldecode, magic quotes) in the metachat code base somehow messed up the site. I'll add it to the list.

Anyway, matthewr: Best Answer.
Although I still think this is way too hard for an 11 year old.

posted by seanyboy 21 November | 04:39

I'm sure I didn't know how to factorise quadratics when I was 11, so I think trial and error must be the method they're looking for.

posted by matthewr 21 November | 04:48

I'm sure I didn't know how to factorise quadratics when I was 11, so I think trial and error must be the method they're looking for.

I *know* it's what they're looking for. My daughter is 11 and she's in the 'top maths group' at a good school. I'm sure she hasn't gotten anywhere near factorising quadratics - I'd have heard all about it.

posted by chuckdarwin 21 November | 04:54

I have specific memories of only just starting to learn the basics of algebra when I was 11 (ah, first form, happy days), so yeah, this is a definitely a 'how do you calculate perimeter and area' and 'how do you use trial and error' question.

posted by chrismear 21 November | 06:09

It really depends on where you go, and unfortunately, how wealthy your parents are. I don't think I learned all of this until around 12 or 13. Meanwhile, my mom's boss' daughter, at the age of 11, is learning calculus.

posted by spiderskull 21 November | 07:36

spiderskull, you probably have a point, there. It's still not fair, and Britain's class system is still in place (educationally, first and foremost).

We had the option of trying for a scholarship to the (very, very pricey and exclusive) girl's college next year... she has the test scores.... but we decided she'd be happier with her friends at the local CofE high school.

posted by chuckdarwin 21 November | 07:45

I'm sure I didn't know how to factorise quadratics when I was 11, so I think trial and error must be the method they're looking for.

Hmm. Data point: I did learn how when I was twelve, in England, which now seems very long ago and far away.

posted by tangerine 21 November | 19:40

← I wish I could knit. || Marvelous Work, Boys →

HOME || REGISTER || LOGIN