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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
00:00
A
B
C
D
E

Difficulty:

  15% (low)

Question Stats:

84% (00:35) correct 16% (00:45) wrong based on 35 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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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, 11:21
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.
R=11 now

Hence new area = 121π

Subtract two areas..

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

Which is 21% .. option B

I hope it was clear with this detailed description

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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 &nbs [#permalink] 26 Nov 2018, 16:26
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If the radius of a circle is increased by 10%, by how much is its are

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