一般的,有:
(n-1)n(n+1)
=n^3-n
{n^3}求和公式:Sn=[n(n+1)/2]^2
{n}求和公式:Sn=n(n+1)/2
1x2x3+2x3x4+3x4x5+.+7x8x9
=2^3-2+3^3-3+...+8^3-8
=(2^3+3^3+...+8^3)-(2+3+...+8)
=[(8*9/2)^2-1]-8*9/2+1
=1260
一般的,有:
(n-1)n(n+1)
=n^3-n
{n^3}求和公式:Sn=[n(n+1)/2]^2
{n}求和公式:Sn=n(n+1)/2
1x2x3+2x3x4+3x4x5+.+7x8x9
=2^3-2+3^3-3+...+8^3-8
=(2^3+3^3+...+8^3)-(2+3+...+8)
=[(8*9/2)^2-1]-8*9/2+1
=1260