mathematical symbol, mathematics, mathematical formula, calculating - mathematical symbol 個影片檔及 b 捲影像00:15
b/w 1949 boy + girl at chalkboard doing addition problem / girl corrects boy's mistake / industrial - mathematical symbol 個影片檔及 b 捲影像00:14
ms pan man writing chinese characters on blackboard / singapore - mathematical symbol 個影片檔及 b 捲影像00:12