九年级数学第二十七章 相似课件平行线分线段成比例定理

I II II II II I3 32 2I I I I则则= =I II II II II II I3 32 2= =I I I II I= =3 31 1= =I I I II I I I= =4 41 1I I I I= =I I I I= =I II II II II II II II II II II I3 32 2I I I I= =I I I I= =5 52 2I I I I= =I I I I= =D DF FE EF FA AC CB BC CD DF FD DE EA AC CA AB BD DE EE EF FA AB BB BC CE EF FD DE EB BC CA AB B则则：, ,/ / /l l/ / /l l综综上上所所述述：若若l l3 32 21 1=上上下下上上下下=下下上上下下上上=E EF FB BC CD DE EA AB B=全全上上全全上上=全全下下全全下下=右右左左右右左左=求求：B BC C. .4 4. .E EF F2 2, ,D DE E3 3, ,，A AB B/ / /l l/ / /l l已已知知：如如图图，l l3 32 21 1=3?42（平行线分线段成比例（平行线分线段成比例定理）定理）6 6BCBC =4 42 2B BC C3 3即即=EFEFDEDEBCBCABAB=/l/l/l/ll l解：解：3 32 21 1Q. .n nm mm mD DF FD DE E求求证证：. .n nm mB BC CA AB B，/ / /l l/ / /l l已已知知：如如图图，l l3 32 21 1=（平行线分线段成比例定理）（平行线分线段成比例定理）. .n nm mm mDFDFDEDE=. .m mn nm mDEDEDFDF即即=，m mm mn nDEDEDEDEEFEF=m mn nDEDEEFEF=n nm mEFEFDEDEBCBCABAB=, ,/l/l/l/ll l3 32 21 1Q：证明?求：求：DE.DE.c.c.EFEFb,b,BCBCa,a,，ABAB/l/l/l/l已知：如图，已知：如图，l l3 32 21 1=DEDEABAB求证：求证：/l/l/l/l已知：如图，已知：如图，l l3 32 21 1=EFEFBCBC=DFDFACACa ab bc c? ?米米. .a abcbc答：答：CFCF长为长为a abcbcCFCFCFCFc cb ba a即即定理）定理）（平行线分线段成比例（平行线分线段成比例CFCFDFDFEBEBAEAEAD/EF/BCAD/EF/BCB B，EFEF9090ABCABCDABDAB解：由题意可知：解：由题意可知：0 0= =A1 1BCBCABAB当当=1 1BCBCABAB当当