【教材分析】教材设计了多个情境,使学生体会用字母表示数的作用。第一个情境是青蛙儿歌,通过儿童熟悉的儿歌,引出用字母表示数,即n只青蛙n张嘴。第二个是妈妈和淘气年龄关系的情境,如果淘气年龄用字母a表示,那么妈妈的年龄可以用a+26表示。第三个是用小棒摆三角形的情境,引导学生用字母表示三角形个数,用a×3表示小棒根数,使学生进一步体会字母表示数的意义。 Teaching material】 【textbook design a number of situations, so that students understand the role of the number of letters. The first scenario is the frog children's songs, children's familiar children's songs, lead to the number of letters, that n frog n mouth. The second is the relationship between mother and naughty age situation, if the naughty age is indicated by the letter a, then the mother's age can use a +26 said. The third is to use a small bar to set the triangle situation, to guide students to use letters to represent the number of triangles, with a × 3 that the number of small rods, so that students further appreciate the significance of the number of letters.