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

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If the radius of a circle is increased by 10%, by how much  [#permalink] New post 23 Oct 2005, 12:58
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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%
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 [#permalink] New post 23 Oct 2005, 13:03
Area of circle A = pi*r^2

increase r by 10%
new area is: pi*(1.1r)^2 = 1.21*pi*r^2

thus area is increased by 21%
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 [#permalink] New post 23 Oct 2005, 16:51
OA is B and here is the explanation:

The formula for the radius of the original circle is

= Pi(radius2)

The formula for the new radius is

= Pi((1.1 * radius)2)
= Pi(1.21 * radius2)

Thus, the area of the circle is increased by 21%.
  [#permalink] 23 Oct 2005, 16:51
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