设小圆半径为r,大圆面积为R
则S阴影=S△大-S△小
=(1/2)R²-(1/2)r²
=(1/2)(R²-r²)
=40
所以R²-r²=80
所以 S大圆-S小圆=πR²-πr²
=π(R²-r²)
=80π
祝你学习愉快!