回複：回複：回複：讓人煩惱的禮金卡問題

來源: 於 2009-01-06 21:45:31 [舊帖] [給我悄悄話] 本文已被閱讀：次

Why not to find r(1-r)/(1-0.9r)'s max?

which would give r = 1 / (1 + sqrt(10%)).