三角形的三边关系（课件）-2023-2024学年苏教版数学四年级下册

三角形的三边关系 家 学校 长申超市 点 线 形 A B C 3 cm 5 cm 4 cm 8 cm 3 cm 4 cm 8 cm 4 cm 5 cm 8 cm 3 cm 8 cm 5 cm 3 cm 4 cm 5 cm 三角形是由3条线段围成的。从下面的4条线段中任选3条，都能围成三角形吗？ 3 cm 4 cm 8 cm 4 cm 5 cm 8 cm 3 cm 8 cm 5 cm 3 cm 4 cm 5 cm 学习任务一： 1.画一画 选择一组线段，用直尺和圆规画一画，并标上数据。 2.想一想 根据作图痕迹，判断这三条线段能否围成三角形。 3.说一说 和组员交流你选了哪一组线段、以及能否围成三角形的原因。 8 cm 3 cm 4 cm 8 cm 3 cm 5 cm 5 cm 4 cm 8 cm 3 cm 4 cm 5 cm 不能围成 能围成 3＋4＜8 3＋5＝8 3＋4＞5 4＋5＞8 5 cm 4 cm 8 cm 3 cm 4 cm 5 cm 3＋4＞5 4＋5＞8 两条短边长度的和大于最长边 能围成 8 cm 3 cm 4 cm 3＋4＜8 8 cm 3 cm 5 cm 3＋5＝8 不能围成 两条短边长度的和小于或者等于最长边 3 cm 4 cm 5 cm 3＋4＞5 8 cm 3 cm 5 cm 4＋5＞8 3＋5＝8 不能围成 能围成 两条短边长度的和大于最长边 8 cm 3 cm 4 cm 3＋4＜8 5 cm 4 cm 8 cm 三条同样长的线段能围成三角形吗？ 3 cm 5 cm 4 cm 8 cm 三条同样长的线段能围成三角形吗？ 从上面四条线段中任选一种，用直尺和圆规画一画，再与同学交流。 学习任务二： 5 5 5 3 3 3 4 4 4 8 8 8 5 5 5 5＋5>5 5＋5>5 5＋5>5 3 cm 4 cm 5 cm 5 cm 4 cm 8 cm 3＋4>5 3＋5>4 4＋5>3 4＋5>8 4＋8>5 5＋8>4 A B 因为 ： 两点之间线段最短 可知 ： AC＋CB＞AB AB＋BC＞AC BA＋AC＞BC 所以 ： 三角形任意两边长度的和大于第三边 C 1.下面哪组线段可以围成一个三角形？为什么？ 2 cm 4 cm 6 cm 2 cm 2 cm 5 cm 2＋4＝6 不能围成 2＋2＜5 不能围成 6 cm 2 cm 5 cm 2＋5＞6 能围成 6＋6＞6 6 cm 6 cm 6 cm 能围成 2cm、2cm、3cm 3cm、3cm、4cm 3cm、3cm、2cm 8 cm 38 cm 25 cm √ √ 8＋14＞20 14＋20＞25 王大伯要钉一个三角形框架，已知两边的长分别是14厘米和20厘米，第三条边的长可能是多少厘米？在合适的答案下面画“√”。 34cm 20cm 14cm 6cm 14cm 7cm 14cm 8cm 14cm 9cm 14cm 12cm 14cm 16cm 14cm 20cm 14cm 14cm 14cm 14cm 第三边 ＜ ＜ 两边之差 两边之和 6cm 34cm 20cm 14cm 1 6cm 1 14cm 1 20cm 34cm 1 点 线 形 A B C ? 新录音 41 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 42 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 43 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 44 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 45 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 46 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 47 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 48 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 49 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 50 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 51 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 52 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 53 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 54 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 55 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 56 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 57 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 58 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 59 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 60 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 61 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 62 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) Lavf58.46.101 新录音 64 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 65 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 63 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 67 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 66 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 71 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 68 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 72 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 69 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 73 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 74 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 75 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 76 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 77 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 78 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 79 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) 新录音 80 com.apple.VoiceMemos (iPhone Version 16.6.1 (Build 20G81)) $$