# Intersections of a Circle with the Four Quadrants

The Ariel University Center in Samaria (Israel) runs an online mathematical olympiad for college students. During the 2009-2010 season they offered several beautiful problems, one of which might appear harder for college students than for younger mathematicians:

Point P = P(a, b) is located in the first quadrant. Consider a circle centered at the point P with radius greater than √a² + b², and denote the area of the part of this circle located in the j-th quadrant (j = 1, 2, 3, 4) by S_{j} . Find _{1} - S_{2} + S_{3} - S_{4}.

The applet below illustrates the problem. (The circle is draggable. Its radius could be changed with the scrollbar at the bottom of the applet.)

What if applet does not run? |

|Activities| |Contact| |Front page| |Contents| |Geometry| |Proofs|

Copyright © 1996-2018 Alexander Bogomolny
Point P = P(a, b) is located in the first quadrant. Consider a circle centered at the point P with radius greater than √a² + b², and denote the area of the part of this circle located in the j-th quadrant (j = 1, 2, 3, 4) by S_{j} . Find _{1} - S_{2} + S_{3} - S_{4}.

What if applet does not run? |

### Solution

Draw two additional lines x = 2a and y = 2b. Together with the axes, the lines split the circle into four areas: four curvilinear rectangles, or cut-off circular segments (equal two-by-two), four curvilinear triangles (all of which are equal - by symmetry), and a rectangle in the middle centered at point P. In the (algebraic) sum _{1} - S_{2} + S_{3} - S_{4}

|Activities| |Contact| |Front page| |Contents| |Geometry| |Proofs|

Copyright © 1996-2018 Alexander Bogomolny68655948