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      歡迎來到原中小學教育資源網!

      《認識三角形》的教學案例與反思

      教學反思 時間:2019-07-04 我要投稿
      【www.51cxty.com - 教學反思】

        1、使學生在觀察、操作、畫圖和實驗等活動中,聯系已有的生活和知識經驗,感受并發現三角形的基本特征,認識底和高,了解三角形的兩邊之和大于第三邊。

        2、使學生在實踐操作、自主探索的過程中,進一步發展空間觀念,鍛煉思維能力。

        教學重點:認識三角形的一些最基本特征。

        教學難點:認識兩邊之和大于第三邊

        教學過程:

        一、導入:

        我們已經初步認識過三角形,生活中你在什么地方看到過三角形?

        出示例題圖,找出圖中的三角形。

        揭示課題:認識三角形

        二、做三角形:

        1、你能想辦法來創造出一個三角形嗎?

        2、全班交流。

        交流:(1)、用小棒擺。

        (2)、在釘子板上圍。

        (3)、用三角板或尺上的其他三角形直接描畫。

        (4)、在紙上分別畫圍起來的三條線段,也能得到一個三角形。

        ……

        3、談話:同學們用自己的方法做出了各種不同的三角形,仔細觀察這些三角形,誰能說說有什么相同之處?

        先分組討論,后集體交流。

        小結:它們都是由三條線段圍成的圖形。

        4、三角形各部分名稱:

        一起動手畫一個三角形,說說各部分的名稱:3個頂點、3條邊、3個角

        三、三邊關系:

        1、是不是所有的三根小棒都能圍成一個三角形?拿出準備好的四根小棒(10厘米、6厘米、5厘米、4厘米),任選三根試一試。

        2、學生操作。

        你們都圍成三角形了嗎?(有的圍成了,有的不能圍成)

        3、談話:通過操作,有的同學用三根小棒圍成了三角形,而有的沒能圍成。

        我們先討論選哪三根小棒圍成了一個三角形?(10厘米、6厘米、5厘米;6厘米、5厘米、4厘米)

        選哪三根小棒不能圍成一個三角形?(10厘米、6厘米、4厘米;10厘米、5厘米、4厘米)

        (課件演示)

        談話:比較三根小棒的長度,你有什么發現?

        提問:根據剛才的實驗,你能發現三角形三條邊的長度有什么關系嗎?

        4、小結:三角形的兩條邊長度的和大于第三邊。

        5、練習:

        你能從1厘米、2厘米、3厘米、4厘米、5厘米、6厘米、7厘米、8厘米這些小棒中選出三根圍成一個三角形嗎?我選7厘米、4厘米的兩根小棒,要圍成一個三角形,你能幫老師選出第三根嗎?

        四、完成書上的想想做做:

        1、在點子圖上畫出兩個三角形:

        指出:畫的時候,要把三角形的三個頂點和點子重合。

        2、下面哪幾組中的三條線段可以圍成一個三角形?為什么?

        在學生交流完后追問第一種情況:那如果老師把2厘米的加上6厘米的,不就變成“大于”4厘米,那就可以圍成三角形了。這樣的判斷對不對?為什么?

        (6厘米是其中最長的一條邊,它單獨一條就比別的兩條都長,所以,要用比較短的邊合起來,然后和最長的比。)

        3、從學校到少年宮有幾條路線?走哪一條路最近?

        請你用今天學得的知識來解釋這一現象。

        五、全課總結

        反思:

        對于四年級的學生來說,三角形一點都不陌生,所以我放手讓學生獨立進行操作,想辦法創造三角形。學生在這種情況下創造性地相出了多種方法,做出了不同的三角形,并在操作過程中體驗到了三角形是由三條線段圍成的圖形,進而認識到三角形的特征。我把較多的時間放在了探究三角形三邊關系方面了,這是本課的一個難點。從“是不是所有的三根小棒都能圍成一個三角形?”,借助了小棒、畫圖等手段,引發學生的主動探究,使學生獲得了一定的數學知識,激發了學習興趣,培養了探索意識。在鞏固練習中的問題,也是幫助學生進一步認識這一難點,并培養學生有序地思考問題。

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