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No. 1
   píng fāng gōng shì n(n+1)(2n+1)/6
   1^2+2^2+3^2+ +n^2=n(n+1)(2n+1)/6( zhù: n^2=n de píng fāng )
   zhèng míng 1 4+ 9+…+ n^2 n(n+1)(2n+1)/6
   zhèng guī cāi xiǎng ):
  1、 n= 1 shí, 1= 1( 1+ 1)( 2×1+ 1) /6= 1
  2、 n= 2 shí, 1+ 4= 2( 2+ 1)( 2×2+ 1) /6= 5
  3、 shè n= x shígōng shì chéng 1+ 4+ 9+…+ x2= x(x+1)(2x+1)/6
   dāng n= x+ 1 shí
  1+ 4+ 9+…+ x2+( x+ 1) 2= x(x+1)(2x+1)/6+( x+ 1) 2
  =( x+ 1) [2( x2)+ x+ 6( x+ 1) ]/6
  =( x+ 1) [2( x2)+ 7x+ 6]/6
  =( x+ 1)( 2x+ 3)( x+ 2) /6
  =( x+ 1) [( x+ 1)+ 1][2( x+ 1) +1]/6
   mǎn gōng shì
  4、 zōng shàng suǒ shù píng fāng gōng shì 1^2+2^2+3^2+… +n^2=n(n+1)(2n+1)/6 chéng zhèng
   zhèng 'èr yòng héng děng shì (n+1)^3=n^3+3n^2+3n+1):
  (n+1)^3-n^3=3n^2+3n+1,
  n^3-(n-1)^3=3(n-1)^2+3(n-1)+1
  ..............................
  3^3-2^3=3*(2^2)+3*2+1
  2^3-1^3=3*(1^2)+3*1+1.
   zhè n děng shì liǎng duān fēn bié xiāng jiā
  (n+1)^3-1=3(1^2+2^2+3^2+....+n^2)+3(1+2+3+...+n)+n,
   yóu 1+2+3+...+n=(n+1)n/2,
   dài rén shàng shì
  n^3+3n^2+3n=3(1^2+2^2+3^2+....+n^2)+3(n+1)n/2+n
   zhěng hòu
  1^2+2^2+3^2+....+n^2=n(n+1)(2n+1)/6
zhèng míng fāng
  zhèng
  ( guī cāi xiǎng ):
  1、 N= 1 shí, 1= 1( 1+ 1)( 2×1+ 1) /6= 1
  2、 N= 2 shí, 1+ 4= 2( 2+ 1)( 2×2+ 1) /6= 5
  3、 shè N= x shígōng shì chéng 1+ 4+ 9+…+ x2= x(x+1)(2x+1)/6
   dāng N= x+ 1 shí
  1+ 4+ 9+…+ x2+( x+ 1) 2= x(x+1)(2x+1)/6+( x+ 1) 2
  =( x+ 1) [2( x2)+ x+ 6( x+ 1) ]/6
  =( x+ 1) [2( x2)+ 7x+ 6]/6
  =( x+ 1)( 2x+ 3)( x+ 2) /6
  =( x+ 1) [( x+ 1)+ 1][2( x+ 1) +1]/6
   mǎn gōng shì
  4、 zōng shàng suǒ shù píng fāng gōng shì 1^2+2^2+3^2+… +n^2=n(n+1)(2n+1)/6 chéng zhèng
   zhèng 'èr
  ( yòng héng děng shì (n+1)^3=n^3+3n^2+3n+1):
  (n+1)^3-n^3=3n^2+3n+1,
  n^3-(n-1)^3=3(n-1)^2+3(n-1)+1
  ..............................
  3^3-2^3=3*(2^2)+3*2+1
  2^3-1^3=3*(1^2)+3*1+1.
   zhè n děng shì liǎng duān fēn bié xiāng jiā
  (n+1)^3-1=3(1^2+2^2+3^2+....+n^2)+3(1+2+3+...+n)+n,
   yóu 1+2+3+...+n=(n+1)n/2,
   dài shàng shì
  n^3+3n^2+3n=3(1^2+2^2+3^2+....+n^2)+3(n+1)n/2+n
   zhěng hòu
  1^2+2^2+3^2+....+n^2=n(n+1)(2n+1)/6
  a^2+b^2=a(a+b)-b(a-b)
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