设球原来的半径为R1,膨胀后的半径为R2.
4πR2²＝2×4πR1²
R2＝√2 R1
4/3 πR2³÷﹙4/3 πR1³﹚＝﹙R2/R1﹚³＝﹙√2﹚³＝2√2.