三角形任意两边之和大于第三边

三角形任意两边之和大于第三边 《三角形任意两边之和大于第三边》教学案例与反思 教材 分析 定性数据统计分析pdf销售业绩分析模板建筑结构震害分析销售进度分析表京东商城竞争战略分析 : “三角形任意两边之和大于第三边”是义务教育课程标准实验教科书小学《数学》(人教板)四年级下册中的教学 内容 财务内部控制制度的内容财务内部控制制度的内容人员招聘与配置的内容项目成本控制的内容消防安全演练内容 。本课是在学生认识了三角形是什么的基础上进一步认识三角形三边的特征。同时，通过这堂课的学习，为学生角的分类提供方法。 教学准备:课件、小棒 教学目标: 1、 通过教师启发，学生经历小组合作、动手实践的过程，体会“三角形任意两边之和大于第三边”。 2、 通过小组合作的形式，增强学生的合作交流意识。 3、 培养学生逻辑思维能力，以及培养学生“猜测—验证—总结”的学习习惯。 教学重点: 理解三角形任意两边之和大于第三边 教学难点: 两边之和等于第三边时不能构成三角形 教学过程: 一、 创设情境 大胆猜测 导语:今天，老师给大家介绍一位新朋友—小明。他正从家里出发赶往学校。请回答从小明家到学校有几条路线,哪一条最近,(指明回答)，【课件出示教材82页例3小明家到学校的路线图】 (1)为什么大家都认为中间这条路最短, 预设 生1:因为第1条和第3条路线拐弯了，绕远路，所以中间这条最近。 生2:我生活中这样走过，中间的这条路线最短。 生3:我在图中通过测量得出中间的这条路线最短。 师总结:同学们结合自己的生活经验谈了自己的感受。那么，如果我们将小明家、邮局、学25Cr-13Ni type or a welding material with high nickel content. Low temperature steels, austenitic stainless steel and heat-resistant corrosion-resistant high alloy steels and austenitic and non-austenite dissimilar steel welded joints shall comply with the following requirements: to ensure penetration and fusion under good conditions, low current, short-arc, welding speed and much faster welding process and interpass temperature should be controlled. High corrosion resistance performance of double-sided welded welds, welded layer in contact with corrosive media finally welded. Low temperature steel welding, suitable for the surface of the weld bead annealing. Check 6.6.1 6.6 welding inspection before welding material and welding materials, must be carried out according to standard acceptance, unqualified persons may be used. Group before the dimensions of various parts of the main structure, Groove sizes, and grooved surface for inspection. Welding Groove and Groove on both sides should be checked before the cleaning quality. Check the welding before welding conditions are consistent with the prescribed conditions, and welding equipment. The drying and cleaning of welding materials to confirm that it meets the requirements. 6.6.2 intermediate welding inspection tack welding after the skull should be cleared checks to welding the defects found shall be removed before they can. Multi-layer welding welding after the each floor should immediately clean the layers, and Visual inspection, found defects in welded to eliminate before they can proceed to the next level. To interrupt the welding seam, should be cleaned and checked before continuing to weld to eliminate welding defects found by the rear. 6.6.3 check after welding seams shall be welded ends immediately after the removal of slag skin, splash, clean weld and weld inspection. Profiles should shape forming beautiful, not undercut, cracked, incomplete fusion, blowhole, slag, splash and other defects. Weld reinforcement should 校这三个位置看成是三角形的三个顶点A、B、C。他们之间的距离看作是三角形的什么,(指名回答) (2)刚才我们都说中间的路比起经过邮局的路要远。也就是说AC边比AB和AC的和要长。 、C位置保持不变，B点可以移动，试想一下，怎样操作使得AB加AC的距离与AC的假如A 距离相差变小, 预设:B点往AC线段靠近。(靠近:可以联系上节课学习三角形高的定义。在这里只要学生能感受靠近的感觉。) 课件演示B点向AC线段近。(B点还未在AC线段上) 现在你会选择哪一线段走到C点,为什么,(指明回答。再次让学生感受三角形两边之和大于第三边。) (3)猜想一下，当B点在哪的时候，使得AB和BC的距离等于AC距离呢, 不知道同学们有没有注意到从刚开始到现在这个图形最大的变化是什么, 生:刚才都是三角形，现在变成了一条直线，不是一个三角形。 你觉得什么时候三边能组成一个三角形,什么时候不能组成一个三角形, 预设生:当两边之和大于第三边的时候能组成一个三角形。两条边之和等于第三边的时候不能组成三角形。 适时提问:只有一组两边的长度的和大于第三条边就能组成一个三角形吗,(例如:长为6cm、3cm、3cm的小棒能摆成一个三角形吗,)此时，启发学生，应是任意三角形两边之和大于第三边。 二、合作交流 探究验证 通过推理，我们认识到任意两边之和大于第三边能组成一个三角形，当两条边之和与第三条相等时不能组成一个三角形。那么，现实生活中，是不是与我们猜测、推理的一样呢,让我们一起动手实践一下吧～ 在这里，老师给每一组都准备了长短不一的小棒，通过小组合作的方式从中选择三根小棒作为三角形的三边，量出它们的长度，拼一拼，最后由小组中的记录员记录实验结果。(小棒的长度:9cm、7cm、6cm、5cm、2cm) 能围成三角形 小棒的长度(单位:厘米) 理由 25Cr-13Ni type or a welding material with high nickel content. Low temperature steels, austenitic stainless steel and heat-resistant corrosion-resistant high alloy steels and austenitic and non-austenite dissimilar steel welded joints shall comply with the following requirements: to ensure penetration and fusion under good conditions, low current, short-arc, welding speed and much faster welding process and interpass temperature should be controlled. High corrosion resistance performance of double-sided welded welds, welded layer in contact with corrosive media finally welded. Low temperature steel welding, suitable for the surface of the weld bead annealing. Check 6.6.1 6.6 welding inspection before welding material and welding materials, must be carried out according to standard acceptance, unqualified persons may be used. Group before the dimensions of various parts of the main structure, Groove sizes, and grooved surface for inspection. Welding Groove and Groove on both sides should be checked before the cleaning quality. Check the welding before welding conditions are consistent with the prescribed conditions, and welding equipment. The drying and cleaning of welding materials to confirm that it meets the requirements. 6.6.2 intermediate welding inspection tack welding after the skull should be cleared checks to welding the defects found shall be removed before they can. Multi-layer welding welding after the each floor should immediately clean the layers, and Visual inspection, found defects in welded to eliminate before they can proceed to the next level. To interrupt the welding seam, should be cleaned and checked before continuing to weld to eliminate welding defects found by the rear. 6.6.3 check after welding seams shall be welded ends immediately after the removal of slag skin, splash, clean weld and weld inspection. Profiles should shape forming beautiful, not undercut, cracked, incomplete fusion, blowhole, slag, splash and other defects. Weld reinforcement should 第一根小棒 第二根小棒 第三根小棒 不能围成三角形 (1)我们发现:_________________________________________ (2)汇报交流 请同学们展示和汇报小组活动的实际记录，说一说有什么发现, 我们发现:三角形任意两边的和大于第三边。两边的和小于或等于第三边不能组成三角形。 对于学生的发现，给予肯定。(结合学生的学习状况，看是否学习判断三条边能组成三角形的捷径。捷径:较短两边之和大于第三边) 三、强化运用 加深理解 、(重回主题图) 1 问:小明家、商店、学校所组成的这个图形中，从小明家去学校，走哪条路最近,为什么,用自己学过的知识回答。 2、基础练习 下面每组中的三条线段能否围成一个三角形,说明理由。 ?3cm、7cm、5cm 25Cr-13Ni type or a welding material with high nickel content. Low temperature steels, austenitic stainless steel and heat-resistant corrosion-resistant high alloy steels and austenitic and non-austenite dissimilar steel welded joints shall comply with the following requirements: to ensure penetration and fusion under good conditions, low current, short-arc, welding speed and much faster welding process and interpass temperature should be controlled. High corrosion resistance performance of double-sided welded welds, welded layer in contact with corrosive media finally welded. Low temperature steel welding, suitable for the surface of the weld bead annealing. Check 6.6.1 6.6 welding inspection before welding material and welding materials, must be carried out according to standard acceptance, unqualified persons may be used. Group before the dimensions of various parts of the main structure, Groove sizes, and grooved surface for inspection. Welding Groove and Groove on both sides should be checked before the cleaning quality. Check the welding before welding conditions are consistent with the prescribed conditions, and welding equipment. The drying and cleaning of welding materials to confirm that it meets the requirements. 6.6.2 intermediate welding inspection tack welding after the skull should be cleared checks to welding the defects found shall be removed before they can. Multi-layer welding welding after the each floor should immediately clean the layers, and Visual inspection, found defects in welded to eliminate before they can proceed to the next level. To interrupt the welding seam, should be cleaned and checked before continuing to weld to eliminate welding defects found by the rear. 6.6.3 check after welding seams shall be welded ends immediately after the removal of slag skin, splash, clean weld and weld inspection. Profiles should shape forming beautiful, not undercut, cracked, incomplete fusion, blowhole, slag, splash and other defects. Weld reinforcement should ?6cm、2cm、2cm ?8cm、4cm、4cm 3、拓展练习 同学们，老师这有一个活动角，角的两边长分别是9cm、7cm，要加一根多长的小棒能够组成一个三角形。(学生任意回答。然后提问:“最小是多少，最大是多少”,课后同学们可以再去研究一下。) 四、 课堂小结 这节课你学会了什么,你有什么收获, 教学反思: 数学学习的本质是使学生获取数学知识，形成数学技能和能力的一种思维活动。没有数学思维，就没有真正的数学学习。作为数学教师应该使学生能够认识并掌握数学思维的方法，如归纳、猜想与验证等等。本课教学自己有如下体会: 一、以情境为出发点，提出合理的猜想，培养学生探索精神。 猜想是依据已有的 材料 关于××同志的政审材料调查表环保先进个人材料国家普通话测试材料农民专业合作社注销四查四问剖析材料 和知识作出符合一定的经验与事实的推测性想象的思维方法。这是一种以联想为中介的合情推理。 在教学中，教师利用情景图(小明上学路线图)首先提出“你怎么知道小明上学走中间这条路最近”这个话题与学生交流。有的学生结合生活经验谈理由，有的学生根据已有的“两点间线段最短”的知识解释原因。接着，学生观察由路线图抽象成的三角形，理解直走的路是这个三角形的一条边，拐了弯的路是这个三角形两条边的和。进而，引导学生联系刚才的判断——直走的路比拐了弯的路近，对“三角形三条边的关系”提出联想或猜想。这样，学生面对“三角形三条边的关系”这一新问题，很自然地得到:三角形两条边的和比另一条边长。在这个过程中，教师采取与学生一起从起点情境出发往上看目标的方法，鼓励学生联系已有知识与经验进行形象的加工和改造，大胆提出新的猜想(或联想)，再由学生想办法来验证猜想。在这个过程中，学生“自己引导思维”，经历“猜想、假定、确定”的过程，体验“冒险、创造、发现”的喜悦。 二、在验证过程中教师适当的启发，促进学生逻辑推理能力的提升。 在学生们确定了验证“三角形两边的和大于第三边”的方法后(方法有:量出三条边的长度再比较，或用小棒摆出三角形直接比较)，教师给学生提供了一张活动记录表，以此帮助学25Cr-13Ni type or a welding material with high nickel content. Low temperature steels, austenitic stainless steel and heat-resistant corrosion-resistant high alloy steels and austenitic and non-austenite dissimilar steel welded joints shall comply with the following requirements: to ensure penetration and fusion under good conditions, low current, short-arc, welding speed and much faster welding process and interpass temperature should be controlled. High corrosion resistance performance of double-sided welded welds, welded layer in contact with corrosive media finally welded. Low temperature steel welding, suitable for the surface of the weld bead annealing. Check 6.6.1 6.6 welding inspection before welding material and welding materials, must be carried out according to standard acceptance, unqualified persons may be used. Group before the dimensions of various parts of the main structure, Groove sizes, and grooved surface for inspection. Welding Groove and Groove on both sides should be checked before the cleaning quality. Check the welding before welding conditions are consistent with the prescribed conditions, and welding equipment. The drying and cleaning of welding materials to confirm that it meets the requirements. 6.6.2 intermediate welding inspection tack welding after the skull should be cleared checks to welding the defects found shall be removed before they can. Multi-layer welding welding after the each floor should immediately clean the layers, and Visual inspection, found defects in welded to eliminate before they can proceed to the next level. To interrupt the welding seam, should be cleaned and checked before continuing to weld to eliminate welding defects found by the rear. 6.6.3 check after welding seams shall be welded ends immediately after the removal of slag skin, splash, clean weld and weld inspection. Profiles should shape forming beautiful, not undercut, cracked, incomplete fusion, blowhole, slag, splash and other defects. Weld reinforcement should 生开展实验活动。在活动中，教师始终将自己当作小组合作学习中的一员，给予适当的启发。例如对于得出结论是“其中两条边的和大于第三边”的小组，教师给出的建议是“其它两条边的和也大于第三边吗,比比看”;对于得出结论是“任意两条边的和大于第三边”的小组，教师鼓励他们大胆思考“只用看哪两条边的和大于第三边就能说明任意两条边的和大于 ;对于用小棒没有围成三角形的小组，教师指导他们从现象中找原因第三边,为什么,” “什么样的情况时三根小棒围不成三角形,”在这个实验活动的过程中(时间大约用了十二分钟)，教师通过亲身参与，针对性的指导，将学生的思维从具体形象思维引向抽象逻辑思维。进而在小组交流汇报中，学生能发现并填补自己认知的空隙，提升思维的优化(用两条较短边的和大于第三边，就能说明任意两边的和大于第三边)。整个过程学生亲历了数学知识的探究与发现，更重要是接受了一次严密的逻辑和数学思维的训练。 本课教学，教师给学生提供了充分地从事数学活动的机会，即学生学会在观察中思考，在思考中猜想，在操作中验证，在交流中发现，并将“学习猜想的方法”与“学习验证的方法”有机结合起来。因此，学生获取的不仅仅是知识本身，更重要的是态度、思想、方法，是一种探究的品质。这样可以提高学生的探究能力，对他们后继知识的学习有较大的影响，也可为其终身学习奠定基础。 25Cr-13Ni type or a welding material with high nickel content. Low temperature steels, austenitic stainless steel and heat-resistant corrosion-resistant high alloy steels and austenitic and non-austenite dissimilar steel welded joints shall comply with the following requirements: to ensure penetration and fusion under good conditions, low current, short-arc, welding speed and much faster welding process and interpass temperature should be controlled. High corrosion resistance performance of double-sided welded welds, welded layer in contact with corrosive media finally welded. Low temperature steel welding, suitable for the surface of the weld bead annealing. Check 6.6.1 6.6 welding inspection before welding material and welding materials, must be carried out according to standard acceptance, unqualified persons may be used. Group before the dimensions of various parts of the main structure, Groove sizes, and grooved surface for inspection. Welding Groove and Groove on both sides should be checked before the cleaning quality. Check the welding before welding conditions are consistent with the prescribed conditions, and welding equipment. The drying and cleaning of welding materials to confirm that it meets the requirements. 6.6.2 intermediate welding inspection tack welding after the skull should be cleared checks to welding the defects found shall be removed before they can. Multi-layer welding welding after the each floor should immediately clean the layers, and Visual inspection, found defects in welded to eliminate before they can proceed to the next level. To interrupt the welding seam, should be cleaned and checked before continuing to weld to eliminate welding defects found by the rear. 6.6.3 check after welding seams shall be welded ends immediately after the removal of slag skin, splash, clean weld and weld inspection. Profiles should shape forming beautiful, not undercut, cracked, incomplete fusion, blowhole, slag, splash and other defects. Weld reinforcement should