Ruler is a plastic ruler commonly used by primary school students to draw straight lines, line segments and lines. Under normal circumstances, when primary school students do math homework, they should draw straight lines, line segments and lines in their exercise books, and all of them should use a ruler. There are three kinds of math exercise books: large, small and minimum, which are 20 cm long and 15 cm wide. In order to draw the longest line in this exercise book, at least.