魯教版9.2-平行線分線段成比例1

平行線分線段成比例A1nmA2A3B1B2B3A1nmA2A3B1B2B3定理的符號(hào)語(yǔ)言

L1//L2//L3=ABDEBCEF(平行線分線段成比例定理)兩條直線被一組平行線所截,所得的對(duì)應(yīng)線段成比例.定理DEFABCL1L2L3L4L5做一做A1A2A3B1B2B3abcmn做一做A1A2A3B1B2B3abcmnlC2C3ABCDEFL1L2L3L4L5L1L2L3ABCDEFL4L5L1L2L3L4L5L1L2L3L5L4L1L2L3L4L5L1L2L3L4L5L4L5L1L2L3ABCDEABACADAE=

DEBC//數(shù)學(xué)符號(hào)語(yǔ)言ABCDEABCDEFL1L2L3L4L5L1L2L3L4L5L1L2L3L5L4L1L2L3L4L5L1L2L3L4L5L1L2L3L4L5L4L5L1L2L3L4L5L1L2L3EABDCABCED

DEBC//ADAEACAB=數(shù)學(xué)符號(hào)語(yǔ)言L1L2L3L4L5L1L2L3L4L5ABCEDABCDE∵DE∥BCADAEACAB=∵∵DE∥BCADAEACAB=∵數(shù)學(xué)符號(hào)語(yǔ)言數(shù)學(xué)符號(hào)語(yǔ)言推論：平行于三角形一邊的直線與其他兩邊相交，截得的對(duì)應(yīng)線段成比例。推論的數(shù)學(xué)符號(hào)語(yǔ)言：∵DE∥BCADAEABAC∴————＝（推論）ABCDEABCEDABCDE————練習(xí)一:1、判斷題:如圖:DE∥BC,下列各式是否正確D:————＝ADAEABAC()C:————＝ADACAEAB()B:————＝ADBDAECE()A:ADAB＝AEAC()ABCED2、填空題:如圖:DE∥BC,已知:2＝——AEAC—5＝——ADAB求:——2—5AEFBC已知：EF//BC,AE=7,EB=5,FC=4.求：AF=?例題解:∵EF∥BCAEAFEBFC∴————＝（推論）7AF54————＝即＝

285—∴AFAEFBC已知：EF//BC,AB=10,AE=6,AF=5.求：FC=?例題解:∵EF∥BCAEAFABAC∴————＝（推論）6510AC————＝即＝

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3—∴ACFC=AC-AF=習(xí)題9.31.ABCDEFl1l2l3(1)457x習(xí)題9.31.ABCDEFl1l2l3(2)567x習(xí)題9.3ABDCE2.(1)如圖:已知DE∥BC,AD=3.2,DB=1.2，AE=2.4，求：EC的長(zhǎng)。(2)AB=5，AD=3，AC=4那么EC的長(zhǎng)。3.已知：DE//AC,求：EABCD4.已知：DE//BC,EF//AB,AD:DB=2:3,BC=20.求：BF=?ABCDEFCB=4，BEAB=AABCDEC達(dá)標(biāo)檢測(cè)題:1、如圖:已知DE∥BC,AB=5,AC=7，

AD=2，求：AE的長(zhǎng)。BDE(A組)(B組)2、已知∠A=∠E=60°求：BD的長(zhǎng)。———23