an = a1+(n-1)d
a1+a3+a5+...+a(2n+1)=518 (1)
a2+a4+a6+...+a(2n)=402 (2)
(1)-(2)
(a1-a2)+(a2-a3)+...+(a(2n-1) +a2n) + a(2n+1) = 116
-nd + a1+2nd =116
a1+nd =116为什么有a1-a2+(a2+a3)(1)-(2)[a1+a3+a5+...+a(2n+1)] -[a2+a4+a6+...+a(2n)] = 518-402(a1-a2)+(a3-a4)+...+(a(2n-1)-a2n) + a(2n+1) = 116