先在立方公式中,取b=1,那么(a+1)3-a3=3a2+3a+1,再让a=1,2,3,…,n-1,n得23-1=3×12+3×1+1,33-23=3×22+3×2+1,43-33=3×32+3×3+1,…,(n+1)3-n3=3×n2+3n+1,再把这些式子相加可得(n+1)3-1=3(12+22+32+…+n2)+3(1+2+3+…+n)+n,从而可证12+22+32+…+n2==.
【解析】
23-13=3×12+3×1+1,
33-23=3×22+3×2+1,
43-33=3×32+3×3+1
┅┅
(n+1)3-n3=3×n2+3×n+1---(6分)
将以上各式分别相加得:
(n+1)3-13=3×(12+22+32+…+n2)+3×(1+2+3…+n)+n
所以:=---------(12分)