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AIBU?

AIBU to disagree with the answer to this maths question?

37 replies

AuntImmortelle · 13/05/2020 10:55

DD answered yes to question 6 on this maths work and we've now been debating if the answer is yes or no.

The mark sheet is on the photo and says no.

I don't think there is enough info to work out the perimeter and that you can't assume the short sides are 2cm.

Argh. Maths people: what is correct??

AIBU to disagree with the answer to this maths question?

OP posts:

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KittenVsBox · 15/05/2020 08:23

I've just asked my (fairly able) Y4 child. He answered instantly, yes, you can work out the answer, if you pull the corner out, you get a rectangle. Slight pause, the answer is 44cm.

BeetrootRocks · 14/05/2020 21:38

Smeerf me too!

2 maths a levels and a physics degree 🤣🤣🤣

A very long time ago but still...

Love Mumsnet. It's a bit much for year 4 though I'd say.

NotMeNoNo · 14/05/2020 21:12

Sorry, the correct answer to the question about Dani is no, dani is wrong, the perimeter can be calculated as pp said. Reverse questions are annoying.

NotMeNoNo · 14/05/2020 21:09

The correct answer is yes

gingerbiscuits · 14/05/2020 21:05

There IS enough info- the 2 unlabelled horizontal sides add up to 12 & the 2 vertical sides add up to 10 so the perimeter is 2x10+2×12 = 44.

MereDintofPandiculation · 14/05/2020 21:02

@CoralFish They don't have to be right angles. It still works if adjacent angles sum to 90 deg and the two "inner" angles are the same - in other words, the shapes is basically a parallelogram with a cut-out, and the the edges of the cut-out are parallel to the edges of the larger shape.

TeenPlusTwenties · 14/05/2020 19:54

mrsB Correct, but there is nothing to say that it is the same 'thickness' in both the horizontal and vertical bars (it looks it, but it doesn't say so), you need to use an x and a y, which still disappear.

mrsBtheparker · 14/05/2020 19:27

If you call the thickness x then it's 12 + 10 +x + x +10-x + 12-x, the x's cancel each other out to leave 44!

MereDintofPandiculation · 14/05/2020 10:37

You don't have to assume. Maths proves it is 2cm. How? I would like to see the proof of that?

But you don't have to assume, because their length is irrelevant.

Imagine the shape is made of string, held in place by pins at each corner. Remove the pin from the inner corner of the shape - it's easy to imagine pulling that corner down to the right until finally the shape becomes a 10 x 12 rectangle. Not a proof, but a practical demonstration if the various proofs seem counter-intuitive.

ChangeThePassword · 14/05/2020 10:27

There is enough information. The short bit plus the long bit that's cut off is the same as the full length.

TeenPlusTwenties · 14/05/2020 10:22

Not No it doesn't.

The short sides could just as easily be 1cm, or 5cm, or one of them 0.5mm and the other 4.8mm.

NotEverythingIsBlackandWhite · 14/05/2020 09:34

"I don't think there is enough info to work out the perimeter and that you can't assume the short sides are 2cm."
You don't have to assume. Maths proves it is 2cm.

helpfulperson · 14/05/2020 08:54

Assuming the angles are right angles then it is possible.

bridgetreilly · 14/05/2020 08:48

It doesn't matter how long the short sides are, the perimeter will be the same.

bestofme21 · 14/05/2020 07:52

Sorry just seen xyzandabc reply! Came to the party a little late

bestofme21 · 14/05/2020 07:50

The answer is 44cm

smeerf · 13/05/2020 12:54

I can't believe I got that wrong! I feel really silly now. I have 2 maths A levels (although they were a long time ago)

qweryuiop · 13/05/2020 12:45

@AuntImmortelle

That's really odd! It's not even an objective until Year 5, but I've found the lesson on their website. White rose resources are fabulous, but they are pretty challenging!

QueenOfToast · 13/05/2020 11:46

@xyzandabc Your explanation was genius - thank you

TeenPlusTwenties · 13/05/2020 11:31

tbf I don't expect a y4 to use algebra.
But in the absence of pen and paper I thought it might help show the logic.

OP, the other thing that can be helpful is to pick up the paper and look at it from the side (like projections of a house). Then it becomes more obvious that the vertical lines must add to 10, and the horizontal ones must add to 12.

Being able to 'see' missing lengths is a useful skill. As is adding bits or splitting things in two or more when finding areas of odd shapes.

chomalungma · 13/05/2020 11:22

Love to see someone using algebra to solve it.

Always the best way

CoralFish · 13/05/2020 11:21

I would say 'Yes' because the angles are not marked as right angles. That is pretty pedantic though, so I wouldn't lose sleep over being told I'm wrong.

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pickingdaisies · 13/05/2020 11:16

You're welcome OP 😊

AuntImmortelle · 13/05/2020 11:13

Year 4!

OP posts:

qweryuiop · 13/05/2020 11:07

@TeenPlusTwenties has a great explanation
The way I would explain to year 6 is that the two short vertical sides have to add up to 10 as they are the same length as the longer vertical side.
The two short horizontal sides have to add up to 12 for the same reason.

Can I be a nosy teacher - I know its white rose, but which year group?

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