S1=5π、S2=8π,相应的圆的半径是R1、R2,球心到这两个截面的距离分分别是d1、d2,球的半径是R,则：R1=√5、R2=√8d1=√(R²－R1²)、d2=√(R²－R2²)则：d1－d2=1√(R²－5)－√(R²－8)=1...取倒数，得：√(R²－5)＋√(R²－8)=3怎么来的？1/[√(R²－5)－√(R²－8)]=1[√(R²－5)＋√(R²－8)]/{[√(R²－5)－√(R²－8)]×[√(R²－5)＋√(R²－8)]}=1[√(R²－5)＋√(R²－8)]/[(R²－5)－(R²－8)]=1√(R²－5)＋√(R²－8)=3