n项和的平方:
(A1+A2+...+An)^2 = (sigma i:1->n)(sigma k:1->n)AiAk
=A1^2+A2^2+...+An^2+ 2A1A2+2A1A3+...+2A1An+ 2A2A3+2A2A4+...+2A2An+ ...+ 2A(n-1)An
ps:sigma是求和符号,i和k是下标.
n项和的平方:
(A1+A2+...+An)^2 = (sigma i:1->n)(sigma k:1->n)AiAk
=A1^2+A2^2+...+An^2+ 2A1A2+2A1A3+...+2A1An+ 2A2A3+2A2A4+...+2A2An+ ...+ 2A(n-1)An
ps:sigma是求和符号,i和k是下标.