The area of a circle is given by the formula A = ðr2, where r is the radius. If increasing the radius of a circle by 1 inch gives the resulting circle an area of 100ð square inches, what is the radius of the original circle?

Answer 1

well, if the original radius is r, we have

π(r+1)^1 = 100π

Or, just note that it's obvious that a radius of 10 gives an area of 100π, so the original radius was 9.