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If the radius of a circle is increased by 10%, by how much is its are

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If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 26 Nov 2018, 04:20
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A
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C
D
E

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84% (00:54) correct 16% (01:00) wrong based on 55 sessions

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Re: If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 26 Nov 2018, 04:44
If r=11 Area= 3.14*(121)

121/100-1= 0.21 = 21%

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Re: If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 26 Nov 2018, 08:56
My solution:

1st (r=2)
pi(r)^2=3.14(4)
pi(r)^2= 12.56

2nd (r=2.2)
pi(r)^2=3.14(4.84)
pi(r)^2=15.20

Calculate change
15.20-12.56=2.64
2.64/12.56=21%


*looking for feedback. I subbed in value 5 too for r and got 20% in the end....
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Re: If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 26 Nov 2018, 10:17
sarajkluz wrote:
My solution:

1st (r=2)
pi(r)^2=3.14(4)
pi(r)^2= 12.56

2nd (r=2.2)
pi(r)^2=3.14(4.84)
pi(r)^2=15.20

Calculate change
15.20-12.56=2.64
2.64/12.56=21%


*looking for feedback. I subbed in value 5 too for r and got 20% in the end....


If r=5 Area= pi*25
If r=5*(1.10)=5.5 Area= pi*30.25
So 30.25/25 -1= 0.21= 21%

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If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post Updated on: 31 May 2019, 08:02
1
Let's take radius (r) = 10

According to the formula πr^2

Original area of a circle be 100π
Now, the new Radius is increased by 10% I.e.
New radius (R) = r+ (10% of r)
R = 10 + (0.1*10) = 10+(1) =
R=11
now

we have new area (bigger one) = 121π


Change in area =
{(new area- original area)/100π (original) } *100%
(121π-100π)/100π*100%

Which is 21% .. option B

I hope it was clear with this detailed description

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Originally posted by Shrey9 on 26 Nov 2018, 11:21.
Last edited by Shrey9 on 31 May 2019, 08:02, edited 2 times in total.
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Re: If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 26 Nov 2018, 16:26
Bunuel wrote:
If the radius of a circle is increased by 10%, by how much is its area increased?

A. 25%
B. 21%
C. 20%
D. 15%
E. 10%



Radius be 10 and 10% increased will be 11

So pi*10^2, 100 *pi and pi*11^2= 121* pi
21/100= 21% option B
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Re: If the radius of a circle is increased by 10%, by how much is its are  [#permalink]

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New post 06 Jan 2020, 06:23
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Re: If the radius of a circle is increased by 10%, by how much is its are   [#permalink] 06 Jan 2020, 06:23
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