If the radius of a circle is decreased by 10%, by what percent is its area decreased

The answer is is 19%.

But I have NO IDEA how to get that answer. Could someone please explain it to me?

10 points for the effort.

input something for the radius say 5
area = pie R(squared)
so 3.14(25) = 78.5
decrease 5 by .1 so = 4.5
area = 3.14 (20.25) = 63.585
63.585/78.5 = .81
1-.81= .19 = 19%

Area of a circle is pie time the square of radius. Say, r=10 then A = 3.14(10)^2 = 314 and reducing 10 by 10% is 9, so A=3.14(9)^2 = 254.34. Divide 254.34 by 314 is 0.81 or 81% therefore 100% - 81% = 19% was the decreased

Area = Pi * R * R
Area 2 = Pi * (R*.9) * (R*.9)
since .9 times .9 is .81 then it was reduced by 19%

pi*r^2 - pi*(.10r)^2

=pi*r^2 * .01*r^2

The circle is decreased by 1%.