Mumsnet Talk

(13 Posts)
NotQuintAtAllOhNo Sun 02-Dec-12 11:20:46

Rectangles A and B have the same area.

A:
3cm
(3x+1)cm

B:
5cm
2x cm

1. What is the area of rectangles A and B
2. What is the perimeter of rectangle B?
3. If rectangle A was enlarged by a scale factor of 3, what would the enlarged area be.
4. If rectangle A was enlarged by a scale factor of 1.5, what would the enlarged area be.

I am as stumped as he is.

FivesAndNorks Sun 02-Dec-12 11:23:34

Area of A is 3(3x+1) (assuming figures are the two sides' lengths)
B is 10x (5 times 2x)

perimeter is adding all sides together

so A: 3+3+(3x+1)+(3x+1) = 6x+8
B: 5+5+2x+2x = 10+4x

Does that make sense

NotQuintAtAllOhNo Sun 02-Dec-12 11:24:48

It makes sense.

Should we not find what the X is?

FivesAndNorks Sun 02-Dec-12 11:25:07

And we know they have the same area, so 3(3x+1)=10x

which means 9x+3 = 10x
so x=3

so the area of each is 30

not totally sure what they mean by scale factor - am guessing they mean each side is extended by that factor, rather than just the overall area - either way, should be calculable

NotQuintAtAllOhNo Sun 02-Dec-12 11:29:10

I am with you up to this point: 3(3x+1)=10x

But I dont see how this is worked out :

which means 9x+3 = 10x
so x=3

(more than 20 years since I saw maths like this)

NotQuintAtAllOhNo Sun 02-Dec-12 11:30:28

Oh I see

9x+3 = 10x

10x - 9 x = 3
x=3

(dim)

NotQuintAtAllOhNo Sun 02-Dec-12 11:31:29

How do we enlarge A by a scale factor of 3? Just multiply?

noblegiraffe Sun 02-Dec-12 11:36:09

Multiply the length of each side by 3 then calculate the new area. Hopefully you'll find the new area is 9 times the original area.

This seems a bit difficult for primary? What level is DS?

NotQuintAtAllOhNo Sun 02-Dec-12 11:43:48

level 5 year 6

noblegiraffe Sun 02-Dec-12 11:48:29

That homework is pushing level 7! No wonder he's finding it hard

NotQuintAtAllOhNo Sun 02-Dec-12 11:58:17

Thanks both of you, we worked it out.

Luckily he grasps it easily once explained.
Level 7? No wonder....

noblegiraffe Sun 02-Dec-12 12:28:29

I would set something like that for Y9 or maybe top set Y8, and expect some of them not to get it.
I suspect that the teacher has misjudged things slightly if that has been set for everyone. Please write a note to say extensive help was needed otherwise the teacher might continue to press ahead thinking 'well Little Johnny got it'.

NotQuintAtAllOhNo Sun 02-Dec-12 12:55:39

They don't all get the same homework luckily, he told me yesterday that he is just below the 4 children in his class which are working towards the level 6 exam, there are 4 different groups in his class. (I dread to think what homework these children get)

Join the discussion

Registering is free, easy, and means you can join in the discussion, get discounts, win prizes and lots more. Register now