geometry

magical cook · Posted: Mon Jan 21, 2008 9:17 am Post subject: geometry

What is the difference in the circumferences of circles A and B?

(1) The difference in the areas of the two circles is 4.
(2) The difference in radii of the two circles is 2.

sankruth · Posted: Mon Jan 21, 2008 9:30 am Post subject:

Radius of larger circle = R
Radius of smaller circle = r

2.Pi.(R-r) = ?

St1

Pi(R^2) - Pi(r^2) = 4

Pi[R^2 - r^2] =4

(R+r)(R-r) = 4/Pi

Not Suff

St2
R-r = 2

Since 2 & Pi are constants, B is SUFF

So B

Stuart Kovinsky · Posted: Mon Jan 21, 2008 9:37 am Post subject:

A slightly different way to think about it:

(1) compares area. However, area is a square function and circumference is a linear function. Therefore, depending on the numbers we select, we'll get different results.

(2) compares radius. Both radius and circumference are linear functions, so if we know the difference in radius, the difference in circumference will be constant (no matter which actual numbers we select).

(2) is suff, 1 is insuff: choose (B).

magical cook · Posted: Mon Jan 21, 2008 10:01 am Post subject:

thank you - I think it's correct. (no OA)