Question

如图以O为圆心的两个同心圆中大圆的弦AB与小圆相交于C、D两点,若AB=8cm,CD=4cm.求圆环的面积

Answer 1

从O作OA垂直于AB,交于E,设OE长为x,大圆半径R,小圆半径r,则有

R^2=4^2+x^2

r^2=2^2+x^2

圆环面积S=∏R^2-∏r^2=∏（R^2-r^2)=12∏