Try this beautiful problem from Geometry based on the Area of a Circle.
Margie's winning art design is shown. The smallest circle has radius 2 inches, with each successive circle's radius increasing by 2 inches. Approximately what percent of the design is black?
Geometry
Area
Circle
Answer:$42$
AMC-8, 2008 problem 25
Pre College Mathematics
Area of the square is \(\pi (r)^2\),where \(r\)=radius of the circle
Can you now finish the problem ..........
Find the total area of the black region........
can you finish the problem........
Given that The smallest circle has radius 2 inches, with each successive circle's radius increasing by 2 inches .
The radius of the 1st circle is 2, So the area is \(\pi(2)^2\)=4\(\pi\) sq.unit
The radius of the 2nd circle is 4, So the area is \(\pi(4)^2\)=16\(\pi\) sq.unit
The radius of the 3rd circle is 6 So the area is \(\pi(6)^2\)=36\(\pi\) sq.unit
The radius of the 4th circle is 8, So the area is \(\pi(8)^2\)=64\(\pi\) sq.unit
The radius of the 5th circle is 10, So the area is \(\pi(10)^2\)=100\(\pi\) sq.unit
The radius of the 6th circle is 12, So the area is \(\pi(12)^2\)=144\(\pi\) sq.unit
Therefore The entire circle's area is 144\(\pi\)
The area of the black regions is \((100\pi-64\pi)+(36\pi-16\pi)+4\pi=60\pi \)sq.unit
The percentage of the design that is black is \((\frac{60\pi}{144\pi} \times 100)\%=(\frac{5}{12} \times 100) \% \approx 42\%\)
AMC - AIME Boot Camp... for brilliant students.
Use our exclusive one-on-one plus group class system to prepare for Math Olympiad
