本文利用數學上有關包絡曲線的概念，嚴謹地推導Moses-Weber三角形區位模型的 等支出曲線。本文證明即使運費費率為固定常數，等支出曲線仍為一條平滑且嚴格凸性的曲 線，而Woodward對Moses 1958年的論文的批評是錯誤的。同時，本文也證明當廠商的廠址離 開商品市場的距離保持不變以及運費費率受運送距離左右時，如果生產函數為齊質，則廠商 的最適區位與產量的變動無關。

　　　　　This paper derives the iso-outlay curve rigorously in the Moses-Weber triangle by using the mathematical concept of the envelope of a family of plane curves. It shows that when the distance of the firm's location from the product market is held constant, the iso-outlay curve is a smooth and strictly convex curve even if transport rates are constant, and Woodwards assertion that the iso-outlay curve was incorrectly represented as a smooth or strictly convex in Mosess 1958 paper is amiss. It also demonstrates that when transport rates are a function of distance, the firms location is independent of output if the production function is homothetic.