若数列{a n }满足 1 a n+1 - 1 a n =d ,(n∈N * ,d为常数),则称数列{a n }为调和数列.已知数列 { 1 x n } 为调和数列,且x 1 +x 2 +x 3 +…+x 20 =200,则x 1 +x 20 =______;x 3 x 18 的最大值等于______.
【简答题】Wal-Mart is not just the world's largest retailer (零售商). It's the world's largest company, which sells in three months what number-two retailer Home Depot sells in a year. Wal-Mart exercises its power...