求助PP2一道数学

STFDreamerUID: 3540572

A rectangular game board is composed of identical squares arranged in a rectangular array of r rows and r+1 columns. The r rows are numbered from 1 through r, and the r+1 columns are numbered from 1 through r+1. If r>10, which of the following represents the number of squares on the board that are neither in the 4th row nor in the 7th column?为什么答案是  r^2-r？

aeolus2014UID: 3547834

r(r+1)-r-(r+1)+1

https://bbs.gter.net/thread-1365746-1-1.html  https://bbs.gter.net/thread-1290906-1-1.html  等帖子都有，这个问题被问过不止三次~LZ要善用搜索啊。