Diagonal Problem Explained in Chinese!
方格圖的對角線所通過的方格數目
understand the formula to count the number of squares passed through by a diagonal of a mxn grid.
reproduced from http://120.101.70.8/longlife/GeoGebra/GGB_Teach/Other/Grid_Number.html by 阿壽工坊---官長壽.
