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OE // AD => DE / CD = AO/AC (1)
OF // BC => FC / Dc = OB /BD (2)
Vì AB // DC => AO / OC = OB /OD
=> AO/(AO+OC) = OB/( OD+OB)
=> AO/AC = OB/ BD (3)
TỪ (1),(2),(3)
=> DE/CD = FC/CD
=> DE = FC (4)
từ o kẻ OH vuông DC
=> S ode = 1/2*DE*OH
S OCf = 1/2*FC*OH
Từ (4) => S ODE = S OCf (dpcm )
Vì \(OE//AD\)
\(\Rightarrow\frac{DE}{DC}=\frac{OA}{CA}\)(1)
\(OF//BC\)
\(\Rightarrow\frac{CF}{CD}=\frac{BO}{BD}\)(2)
\(AB//CD\)
\(\Rightarrow\frac{AO}{AC}=\frac{BO}{BD}\)(3)
Từ (1) ; (2) và (3)
\(\Rightarrow\frac{DE}{DC}=\frac{CF}{DC}\)
\(\Rightarrow DE=CF\)
Mà trong các tam giác ODE và OCF có 1 cặp cạnh = nhau , có cùng chiều cao hạ từ đỉnh O tới cặp cạnh đó nên
\(\Rightarrow S_{ODE}=S_{OFC}\)(đpcm)
AB//CD theo Thales có:
\(\frac{OA}{OC}=\frac{OB}{OD}\Leftrightarrow\frac{OA}{OB}=\frac{OC}{OD}=\frac{OA+OC}{OB+OD}=\frac{AC}{BD}\Rightarrow\frac{OC}{AC}=\frac{OD}{BD}\left(1\right)\)
OE//AD nên \(\frac{OC}{AC}=\frac{CE}{CD}\left(2\right)\) và OF//BC nên \(\frac{OD}{BD}=\frac{DF}{DC}\left(3\right)\)
(1),(2) và (3) có: \(\frac{CE}{CD}=\frac{DF}{CD}\Rightarrow CE=DF\Leftrightarrow CF+EF=DE+EF\Leftrightarrow CF=DE\)
Suy ra \(S_{ODE}=S_{OCF}\) ( chung đỉnh nên cùng chiều cao và 2 đáy bằng nhau)
Xét ΔOCD có AB//CD (gt), theo đ/lí Ta-lét có: \(\frac{OA}{AC}=\frac{OB}{BD}\left(1\right)\)
Xét ΔADC có OE//AD (gt), theo đ/lí Ta-lét có: \(\frac{OA}{AC}=\frac{DE}{DC}\left(2\right)\)
Xét ΔBCD có OF//BC (gt), theo đ/lí Ta-lét có: \(\frac{OB}{BD}=\frac{CF}{CD}\left(3\right)\)
Từ (1), (2), (3) => \(\frac{DE}{CD}=\frac{CF}{CD}\) => DE = CF
=> SODE = SOCF (2 Δ có chung đường cao hạ từ O, đáy DE = CF)
Bạn tự vẽ hình nhé
Xét \(\Delta ACD\) có OE // CD(gt)
=> \(\dfrac{OE}{DC}=\dfrac{AO}{AC}\left(1\right)\)
Xét \(\Delta BCD\) có OF // CD (gt)
=> \(\dfrac{OF}{DC}=\dfrac{BF}{FC}\left(2\right)\)
Mặt khác AB // CD nên \(\dfrac{AO}{AC}=\dfrac{BF}{FC}\left(3\right)\)
Từ \(\left(1\right),\left(2\right),\left(3\right)\)
=> \(\dfrac{OE}{DC}=\dfrac{OF}{DC}\) => OE = OF
Tam giác ABD có OE//AB
=>DO/DB = OE/AB (Theo hệ quả Đlý Ta-lét) (1)
Tam giác ABC có OF//AB
=>CO/CA = OF/AB (Theo hệ quả Đlý Ta-lét) (2)
Tam giác ABO có CD//AB
=>OD/OB = OC/OA (Theo hệ quả Đlý Ta-lét)
=> OD/(OB+OD) = OC/(OA+OC) hay OD/DB=CO/CA (3)
Từ (1) (2) và (3)
=> OE/AB = OF/AB
=> OE = OF (đpcm.)
Áp dụng hệ quả của định lí Ta – lét cho OE//DC,
OF//DC và AB//DC ta được:
Điều phải chứng minh.
Áp dụng hệ quả của định lí Ta – lét cho OE//DC,
OF//DC và AB//DC ta được:
Điều phải chứng minh.
Xét tam giác ADC có EO // CD nên :
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(Hệ quả định lí ta- let).
Xét tam giác BDC có OF // CD nên:
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( hệ quả định lí Ta- let)
Xét tam giác ABC có OF // AB nên theo định lí Ta – let :
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Từ (1); (2); (3) suy ra:
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format('truetype')%3Bfont-weight%3Anormal%3Bfont-style%3Anormal%3B%7D%3C%2Fstyle%3E%3C%2Fdefs%3E%3Ctext%20font-family%3D%22math19a3633cbb697f9e38db19343fa%22%20font-size%3D%2216%22%20text-anchor%3D%22middle%22%20x%3D%2210.5%22%20y%3D%2216%22%3E%26%23x21D2%3B%3C%2Ftext%3E%3Ctext%20font-family%3D%22Arial%22%20font-size%3D%2216%22%20font-style%3D%22italic%22%20text-anchor%3D%22middle%22%20x%3D%2226.5%22%20y%3D%2216%22%3EO%3C%2Ftext%3E%3Ctext%20font-family%3D%22Arial%22%20font-size%3D%2216%22%20font-style%3D%22italic%22%20text-anchor%3D%22middle%22%20x%3D%2237.5%22%20y%3D%2216%22%3EE%3C%2Ftext%3E%3Ctext%20font-family%3D%22math19a3633cbb697f9e38db19343fa%22%20font-size%3D%2216%22%20text-anchor%3D%22middle%22%20x%3D%2252.5%22%20y%3D%2216%22%3E%3D%3C%2Ftext%3E%3Ctext%20font-family%3D%22Arial%22%20font-size%3D%2216%22%20font-style%3D%22italic%22%20text-anchor%3D%22middle%22%20x%3D%2270.5%22%20y%3D%2216%22%3EO%3C%2Ftext%3E%3Ctext%20font-family%3D%22Arial%22%20font-size%3D%2216%22%20font-style%3D%22italic%22%20text-anchor%3D%22middle%22%20x%3D%2281.5%22%20y%3D%2216%22%3EF%3C%2Ftext%3E%3C%2Fsvg%3E)
(đpcm)
