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# GCSE similar shapes

Maths and statistics discussion, revision, exam and homework help.

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1. GCSE similar shapes
c) Area of triangle ABC
------------------- = k
Area of triangle CDE

Work out the value of k.
2. Re: GCSE similar shapes
Area of triangle ABC/Area of triangle CDE = k

Work out the value of k
3. Re: GCSE similar shapes
help please
4. Re: GCSE similar shapes
Find the scale factor of the two shapes and then you should be able to find the size of each length. I think this is the way to do it.
5. Re: GCSE similar shapes
(Original post by rapdog)
help please
I assume you have already shown with angle facts that these are mathematically similar shapes.

You should know that for mathematically similar shapes the ratio or their areas is equal to the square of the ratio of corresponding sides.
So if the lengths are 3 times bigger then the area would 9 times bigger
6. Re: GCSE similar shapes
the scale factor is 1.5^2

what do i do next?
7. Re: GCSE similar shapes
(Original post by rapdog)
the scale factor is 1.5^2 what do i do next?
Find the area of the triangle by cutting it in the middle and using pythagoras to find the vertical height of the triangle and then with the length of the vertical height you can find the area of the triangle. I think this is the right way. The scale factor is 1.5 not 1.5^2
Last edited by shyamshah; 10-05-2012 at 20:17.
8. Re: GCSE similar shapes
thanks
9. Re: GCSE similar shapes
(Original post by rapdog)
the scale factor is 1.5^2

what do i do next?
you have finished

The big triangle is 1.5^2 times bigger than the little one
The area of big triangle divided by the area of the little one = 1.5^2
10. Re: GCSE similar shapes
(Original post by gdunne42)
you have finished

The big triangle is 1.5^2 times bigger than the little one
The area of big triangle divided by the area of the little one = 1.5^2
silly me!
11. Re: GCSE similar shapes
(Original post by shyamshah)
Find the area of the triangle by cutting it in the middle and using pythagoras to find the vertical height of the triangle and then with the length of the vertical height you can find the area of the triangle. I think this is the right way. The scale factor is 1.5 not 1.5^2
Try drawing a square of length 2 cm and another of length 3 cm.
How many times longer is a side? How many times bigger is the area ?

They are not isosceles triangles so your suggestion isn't going to work.
You could use the Cosine Rule to find an angle and then the formula 0.5 x abSinC to find and compare the areas.
12. Re: GCSE similar shapes
(Original post by gdunne42)
Try drawing a square of length 2 cm and another of length 3 cm. How many times longer is a side? How many times bigger is the area ? They are not isosceles triangles so your suggestion isn't going to work. You could use the Cosine Rule to find an angle and then the formula 0.5 x abSinC to find and compare the areas.
I thought i did it wrong but wasn't sure, pythagoras works on all triangles and not just isoceles though i thought.
Thank you for clearing it up.
13. Re: GCSE similar shapes
(Original post by shyamshah)
I thought i did it wrong but wasn't sure, pythagoras works on all triangles and not just isoceles though i thought.
Thank you for clearing it up.
Only on right angled triangles.
14. Re: GCSE similar shapes
(Original post by shyamshah)
I thought i did it wrong but wasn't sure, pythagoras works on all triangles and not just isoceles though i thought.
Thank you for clearing it up.
Pythag only works on right angled triangles but if they were isosceles you could cut them in half to make 2 identical right angled triangles.
Last edited by gdunne42; 11-05-2012 at 08:44.
15. Re: GCSE similar shapes
(Original post by Math12345)
Only on right angled triangles.
Oh ok thank you, I really should know that.

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Last updated: May 10, 2012
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