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# Semi-Circle Tweet

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1. Semi-Circle
I need to find the perimeter of this.

2. Re: Semi-Circle
Am I meant to be using c=pi*d
3. Re: Semi-Circle
(Original post by zed963)
Am I meant to be using c=pi*d
Yes, that's where you start
4. Re: Semi-Circle
(Original post by zed963)
Am I meant to be using c=pi*d
Yes, that will give you the circumference of the whole circle.

But here you have half a circle. So how can you use C to find the perimeter?
5. Re: Semi-Circle
(Original post by notnek)
Yes, that will give you the circumference of the whole circle.

But here you have half a circle. So how can you use C to find the perimeter?
Divide it by 2
6. Re: Semi-Circle
(Original post by zed963)
Divide it by 2
Yes and then add on the diameter to find the perimeter.
7. Re: Semi-Circle
Thank you notnek.
8. Re: Semi-Circle
How would I do this question

I've worked it out now.
Last edited by zed963; 08-08-2012 at 19:31.
9. Re: Semi-Circle
(Original post by zed963)
How would I do this question
If you wrap the cotton around the reel one time then the length will be equal to the circumference of the reel.

So if you wrap it around 500 times then the length is...
10. Re: Semi-Circle
(Original post by notnek)
If you wrap the cotton around the reel one time then the length will be equal to the circumference of the reel.

So if you wrap it around 500 times then the length is...
40m Well not exactly 40 its a bit over.

What I did was * the radius by 2 and then by pi

Then times the circumference by 500 and then divide it by 100 to get the answer in metres.
11. Re: Semi-Circle
(Original post by zed963)
40m Well not exactly 40 its a bit over.

What I did was * the radius by 2 and then by pi

Then times the circumference by 500 and then divide it by 100 to get the answer in metres.
Yes that's right.

I'm not sure if you needed my help for that one
12. Re: Semi-Circle
(Original post by notnek)
Yes that's right.

I'm not sure if you needed my help for that one
Yeah sometimes I just post the question because I can't think of how to get the answer and sometimes I put it up but within 5 minutes of putting it up I work it out so it's quite bizarre.
13. Re: Semi-Circle
What am I doing wrong?

I worked out the area of the circle and then the rectangle and then I subtracted the rectangle from the circle but I'm not getting the right answer.
14. Re: Semi-Circle
(Original post by zed963)
What am I doing wrong?

I worked out the area of the circle and then the rectangle and then I subtracted the rectangle from the circle but I'm not getting the right answer.
Your method is correct.

15. Re: Semi-Circle
(Original post by notnek)
Your method is correct.

Well to find the area of the circle I used the formula a=pir^2

I know that the diameter of the circle is 10 therefore the radius will be half the diameter which is 5

substituting the value into the formula I get 78.54

I then work out the area of the rectangle which is 48

78.54-48=30.54cm^2

but the answer in the book is 20.5cm^2
16. Re: Semi-Circle
I got...

((25 * pi) / 2) + 25 = 64.26990816987241548078304229099 4

25 * pi gives you the perimeter of a circle which a diameter of length 25. Halving that number gives you the length of the curved edge of the semi-circle. Adding 25 adds the length of the bottom-line of the semi-circle, giving the length of semi-circle's total perimeter.

If I've made a mistake here I'd appreciate a correction, but this reasoning seems to make sense to me.
17. Re: Semi-Circle
(Original post by zed963)
Well to find the area of the circle I used the formula a=pir^2

I know that the diameter of the circle is 10 therefore the radius will be half the diameter which is 5

substituting the value into the formula I get 78.54

I then work out the area of the rectangle which is 48

78.54-48=30.54cm^2

but the answer in the book is 20.5cm^2