jakolik wrote:

Hi,

The area of an equilateral triangle is 9\sqrt{3}. What is the area of it circumcircle.

A.10PI

B.12PI

C.14PI

D.16PI

E.18PI

Is this a GMAT question?

regards,

Jack

Edited the stem: it should be "The area of an equilateral triangle is

9\sqrt{3}", (otherwise answer would be

{4\sqrt{3}\pi}, which is not listed).

The area of an equilateral triangle equals to

A=a^2\frac{\sqrt{3}}{4}=9\sqrt{3}, where

a is a side of a triangle -->

a=6.

The radius of the circumscribed circle for equilateral triangle is

R=a\frac{\sqrt{3}}{3}=2\sqrt{3}.

Area of a circle is

\pi{R^2}=12\pi.

Answer: B.

Or if you don't remember the formula for radius you should recall another property: radius of a circumscribed circle is 2/3 of the median, which for equilateral triangle is hight too. So calculate height from side and get the radius from it.

For more on this issue please check Triangles Chapter of the Math Book (link in my signature).

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