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Urgent Y7 maths homework help!!!!

(23 Posts)
Jux Tue 04-Jan-11 19:57:18

DD's homework is to find the area of a trapezium with the top measuring 8cm and the bottom 14cm. That's it; no further measurements.

Is it possible to work this out without the height, and if so, how? (I am about to embark on an OU basic maths course, so I may be able to actually help her in a few months' time!)


lal123 Tue 04-Jan-11 20:01:02

here but you need the height of the trapezium too?

Talkinpeace Tue 04-Jan-11 20:07:37

if its symmetrical, then you can work out the length of the base of the triangle - 3cm
so chances are its a 3,4,5
so the height is 4
so the area is 3 x 4 + 8 x 4 = 44cm sq

Jux Tue 04-Jan-11 20:23:56

Exactly lal123 grin thanks anyway.

Talkinpeace, I'll go with that; not sure how dd will feel about "making up" the height but at least it'll show she knows how to do it.

Talkinpeace Tue 04-Jan-11 20:25:58

Not making it up.
3:4:5 triangles are the classic form.
You know its a right angle triangle.
You know one of the sides by the right angle is length 3
therefore the other two sides HAVE to be 4 (the vertical) and 5 (the hypotenuse)

Talkinpeace Tue 04-Jan-11 20:27:01

PS checked it with DH and he came up with the same number I did...

noblegiraffe Tue 04-Jan-11 21:23:26

I doubt they would expect a Y7 to know about Pythagoras, more likely a mistake in the question.

mnistooaddictive Tue 04-Jan-11 21:26:41

it doesn't have to be a 3 4 5 triangle. Sounds like a mistake to me.

Talkinpeace Tue 04-Jan-11 21:27:25

DD did 3 4 5 triangles in extended maths in year 6
they then did bits of it in geometry at the start of year 7
they are SO elegant that they are a hook to get the kids interested in mathematics as against numeracy

Jux Tue 04-Jan-11 21:43:53

DD has done hypoteneuse, though I don't know about the classic form of a triangle. I'm sorry Talkinpeace, I wasn't being rude just thinking that that was what dd would think.

Out of interest, why does the vertical have to be 4? Couldn't it be 12? or any other length?

I thought dd must have forgotten to write down the height, or that her teacher had made a mistake (though that was less likely). Having read about the classic triangle now, I suspect her teacher expected the kids to be able to make the assumption.

Anyway, dd accepted that she should assume the height as 4 and has done her calculation (correctly as it happens; she's actually quite good at maths though she thinks she isn't).

Thank you all. (I shall come back no doubt, again and again and again!)

noblegiraffe Tue 04-Jan-11 21:44:33

If a triangle has a base of 3 and is a right-angled triangle, the other sides could be e.g. 4.5 and 5.4 (rounded) or lots of other numbers. They don't have to be 4 and 5.

Talkinpeace Tue 04-Jan-11 21:56:31

they could indeed : a sqd + b sqd = c sqd
but the fact that the difference between the two lengths is 6 making the triangle base 3 and it being a year 7 question hints at the most elegant answer

happy to be proven wrong when OP's DD checks at school ;-)

noblegiraffe Tue 04-Jan-11 22:05:51

It being a Y7 question hints at there being an error in the question. Teaching the formula for the area of a trapezium is tricky enough.

And I hope no maths teacher would set a question which required students to guess at a length because it is impossible to work out. An answer being elegant does not make it correct.

lal123 Tue 04-Jan-11 22:13:50

I think there's a mistake in the question too - I think best bet is to put in his answer that it is impossible to tell without the height and NOT to make up an answer

Jux Wed 05-Jan-11 09:19:48

Ah, lal123, you could be right there! I wish I'd checked this thread before I went to bed, but I didn't. I'm sure dd would have preferred not to have done any calculations at all and just put in a note telling her teacher it was impossible to do! I said that if she assumed the height was 4 then she could at least show that she knew how to calculate the area, which seems like a fairly sensible way to go.

It had to be done for today, but she - apparently - doesn't actually have a maths lesson today hmm I have no idea when she's going to get it back. I'm intrigued now.

Anyway, I think the most likely situation is that she simply didn't copy the question down properly grin

HaveAHappyNewJung Wed 05-Jan-11 09:31:15

I certainly hope the teacher wasn't expecting them to assume/guess the height - very bad form <maths student here> I would complain if so.

In your DD's situation I would have done the calculations but using h as the unknown height so the area would be

8h (rectangle) + 3h (two triangles) = 11h

Jux Wed 05-Jan-11 10:03:26

Oh bother, you're quite right. Aaaaaaargh. Why oh why didn't I think of algebra? Bother bother bother.

HaveAHappyNewJung Wed 05-Jan-11 12:56:34

Dont beat yourself up jux - its not actually your homework, it's your DD's

Jux Wed 05-Jan-11 22:06:50

I know, but in the light of the maths course I'm starting in Feb, I feel that I would do better going back to primary!

(I asked her, when she got home today, if she would like to add an alternative solution using h, but she wasn't interested. Surprise!grin)

Talkinpeace Wed 05-Jan-11 22:54:12

h is elegant too!
Please tell us what the actual answer is when the work gets marked.

Jux Fri 07-Jan-11 19:17:35



No one got that question right as the teacher had forgotten to tell them that the height was - wait for it - 6 !!!!!!!

I am also happy to announce that no one in the class thought of using h either.

Cheers Teach!wink

Talkinpeace Fri 07-Jan-11 20:49:11

delighted to hear the answer
SUCH a shame that the teacher canot be penalised for setting dud homework.
Time to write 66 in big letters on some sheets of paper for him!!!

Jux Fri 07-Jan-11 22:34:37

That would be fun wouldn't it? As it is, dd has let it be known that she is not going to let him forget it in a hurry. I am so tempted to suggest that she make a large sign with 666 on it to hold up in class, or to bribe the whole class to keep whispering 6 when his back's turned. No, I must leave the poor man alone and not encourage evilness in my thoroughly pure and innocent daughter, who has never had a bad thought in her head.

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