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maths-perimeter

1 18 hours ago Report post
The circle (diameter 16cm) and the semicircle, including its diameter, have the same perimeter. Work out the radius, r, of the semicircle.
So far...
I worked out the perimeter of the circle being 16pie and I know that this would be the same as the perimeter of the semicircle.

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Reply 1

ckfeister

Well, if the diameter is 16cm then half it for the radius..

16/2 = 8cm.. don't they teach you that raidus is half of a circle while diameter is the full length?

Reply 2

neharajput

Original post by ckfeister

Well, if the diameter is 16cm then half it for the radius..

16/2 = 8cm.. don't they teach you that raidus is half of a circle while diameter is the full length?

yeah I know that but that's not what the question is asking... there's a circle and a semicircle with the same perimeter, which would mean their radius is different from one another

Reply 3

thefatone

Original post by neharajput

yeah I know that but that's not what the question is asking... there's a circle and a semicircle with the same perimeter, which would mean their radius is different from one another

work out perimeter of circle and semicircle
set them equal to each other solve for radius of semicircle

so you'll whap you fraction in your calculator and you should get a right answer but i don't guarantee i will be right so @Student403 and @Zacken

(edited 8 years ago)

Reply 4

Student403

Please don't post full solutions but that's not correct because you've added 16 to the pi * d / 2 instead of just d. 16 is not the diameter. d is.

Reply 5

thefatone

Original post by Student403

Please don't post full solutions but that's not correct because you've added 16 to the pi * d / 2 instead of just d. 16 is not the diameter. d is.

oh yea.... lemme correct it then

Reply 6

Student403

Original post by thefatone

oh yea.... lemme correct it then

But you're still giving the full solution which is against the forum guidelines

Reply 7

thefatone

Original post by Student403

But you're still giving the full solution which is against the forum guidelines

if i give a worded walkthrough of how to do it then put answer in spoiler is that ok?

Reply 8

Student403

Original post by thefatone

if i give a worded walkthrough of how to do it then put answer in spoiler is that ok?

Worded walkthrough without the answer would be appropriate

Moved to maths.

Original post by Student403

Worded walkthrough without the answer would be appropriate

I tried doing the perimeter of the semicircle= perimeter of the circle and solved it, but then it didn't work when I substituted it in, which is why I'm confused...

Reply 11

Student403

Original post by neharajput

I tried doing the perimeter of the semicircle= perimeter of the circle and solved it, but then it didn't work when I substituted it in, which is why I'm confused...

Mind showing me your workings? I can tell you where you went wrong :smile:

Reply 12

Zacken

Original post by neharajput

I tried doing the perimeter of the semicircle= perimeter of the circle and solved it, but then it didn't work when I substituted it in, which is why I'm confused...

Circle: perimeter

2 \pi r = 2\pi \times 8 = 16\pi

.

Semi-circle: perimeter

\frac{2 \pi r}{2} = \pi r

, but including the diameter of the semi-circle gives us (perimeter plus diameter):

\pi r + 2r

Original post by ckfeister

Well, if the diameter is 16cm then half it for the radius..

16/2 = 8cm.. don't they teach you that raidus is half of a circle while diameter is the full length?

Bit rich coming from someone who didn't even understand the question and flopped incredibly.

Reply 13

Alexion

I worked out the perimeter of the circle being 16pie

Reply 14

neharajput

Original post by Zacken

Circle: perimeter

2 \pi r = 2\pi \times 8 = 16\pi

.

Semi-circle: perimeter

\frac{2 \pi r}{2} = \pi r

, but including the diameter of the semi-circle gives us (perimeter plus diameter):

\pi r + 2r

.

Bit rich coming from someone who didn't even understand the question and flopped incredibly.

I got those formulas previously and worked it out as the radius being 5.3 recurring?
Thank you so much for your help btw, much appreciated :smile:

Reply 15

neharajput

Original post by Student403

Mind showing me your workings? I can tell you where you went wrong :smile:

Perimeter of the circle= 16pie
(pie x radius) + (2 x radius) = 16pie
Divide both sides by pie...
radius + 2radius= 16
r=16/3
r=5.3 recurring

Reply 16

Student403

Original post by neharajput

Perimeter of the circle= 16pie
(pie x radius) + (2 x radius) = 16pie
Divide both sides by pie...
radius + 2radius= 16
r=16/3
r=5.3 recurring

incorrect here. What happens when you divide 2radius by pi?

It's pi btw not pie :tongue:

Reply 17

neharajput

Original post by Student403

incorrect here. What happens when you divide 2radius by pi?

It's pi btw not pie :tongue:

ohhh sorry loooool omg:' :wink:

anyways...
when you divide 2 radius by pi you get 2 radius/ pi
but then if u multiply everything by pi to get rid of that your back to where you were before, which is what I'm unsure about...

(edited 8 years ago)

Reply 18

ckfeister

Original post by Zacken

Circle: perimeter

2 \pi r = 2\pi \times 8 = 16\pi

.

Semi-circle: perimeter

\frac{2 \pi r}{2} = \pi r

, but including the diameter of the semi-circle gives us (perimeter plus diameter):

\pi r + 2r

.

Bit rich coming from someone who didn't even understand the question and flopped incredibly.

Oh look, its that crybaby from " Improve my grades " about his lazy background thinking it was hell.

Reply 19

Zacken

Original post by neharajput

ohhh sorry loooool omg:' :wink:

anyways...
when you divide 2 radius by pi you get 2 radius/ pi
but then if u multiply everything by pi to get rid of that your back to where you were before, which is what I'm unsure about...