立方根 ：
3√0 = 0
3√1 = 1
3√2 = 1.25992104989487
3√3 = 1.44224957030741
3√4 = 1.5874010519682
3√5 = 1.7099759466767
3√6 = 1.81712059283214
3√7 = 1.91293118277239
3√8 = 2
3√9 = 2.0800838230519
3√10 = 2.15443469003188
3√11 = 2.22398009056932
3√12 = 2.28942848510666
3√13 = 2.35133468772076
3√14 = 2.41014226417523
3√15 = 2.46621207433047
3√16 = 2.51984209978975
3√17 = 2.57128159065824
3√18 = 2.6207413942089
3√19 = 2.66840164872194
3√20 = 2.71441761659491
3√21 = 2.75892417638112
3√22 = 2.80203933065539
3√23 = 2.84386697985157
3√24 = 2.88449914061482
3√25 = 2.92401773821287
3√26 = 2.96249606840737
3√27 = 3
3√28 = 3.03658897187566
3√29 = 3.07231682568585
3√30 = 3.10723250595386
1～30里，只有3个数的立方根是整数，分别是1、8、27。
