【设计意图】小学阶段数学概念几乎都能找到现实模型,数学教学重在唤醒学生已有的认知和经验。学生对周长已有一定的感性认知,如:剪图案、量腰围等,基于学生对周长已有的生活经验和认知水平,通过“剪小鱼图案”这一学生熟知的生活情境,激活学生对周长的感知,再引导学生通过观察、比较、描 Design intent Almost all of the mathematical concepts in primary school can find a realistic model, and mathematics teaching focuses on awakening students’ existing cognitions and experiences. Students have a perceptual perception of the perimeter, such as: cutting patterns, waist circumference, etc., based on the students have the experience of life on the perimeter and cognitive level, through the Life situation, activate students’ perception of perimeter, and then guide students through observation, comparison, description