Page 88 - Engineering
P. 88
ﺍﳌﺜﻠﺜﺎﺕ ٧٥
وﺑﺎﳌﺜﻞ .△ABC ∼ △BDC ،إذن .△ADB ∼ △BDC ،ﻣﻦ ذﻟﻚ ﻧﺮى أن
. AD = DBإذن.(BD)2 = (AD)(DC ) = 2 × 4 = 8 ،
BD DC
وﺬا ﻳﻜﻮن .BD = 8 = 2 2
ﻣﻠﺤﻮﻇﺔ :ﳝﻜﻦ اﺳﺘﺨﺪام اﳌﺜﻠﺜﺎت اﳌﺘﺸﺎﺔ ﰲ اﳌﺜﺎل ) (١٥ﻹﺛﺒﺎت ﻣﱪﻫﻨﺔ ﻓﻴﺜﺎﻏﻮرس
ﻋﻠﻰ اﻟﻨﺤﻮ اﻟﺘﺎﱄ:
ﻧﻔﺮض أن .DC = y ،AD = x ،AC = b ،BC = a ،AB = cﻣﻦ
△ABC ∼ △ADBﳒﺪ أن AB = ACأي أن . c = bوﺬا ﻓﺈن
xc AD AB
.c2 = bxوﻣﻦ △ABC ∼ △BDCﳒﺪ أن . BC = ACأي أن .a = b
ya DC BC
وﺬا ﻓﺈن .a2 = byاﻵن.c2 + a2 = b(x + y) = b2 ،
،AD = DB = 5 ﻣﺜﺎل ) :[MAΘ 1787] (١٦ﰲ اﻟﺸﻜﻞ أدﻧﺎﻩ ،ﻟﺪﻳﻨﺎ
.AED = 90° ،AE = 4 ،EC = 8ﺟﺪ .BC
A
E
D
BC
اﻟﺤﻞ :ارﺳﻢ BHﻳﻮازي DEوﻳﻘﻄﻊ ACﰲ اﻟﻨﻘﻄﺔ .H