AIBU to disagree with the answer to this maths question?

[AuntImmortelle]

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??

[NotEverythingIsBlackandWhite]

"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.

[TeenPlusTwenties]

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.

[ChangeThePassword]

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

[MereDintofPandiculation]

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.

[mrsBtheparker]

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!

[TeenPlusTwenties]

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.

[MereDintofPandiculation]

@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.

[gingerbiscuits]

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.

[NotMeNoNo]

The correct answer is yes

[NotMeNoNo]

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.

[BeetrootRocks]

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.

[KittenVsBox]

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.