[Toán 9]

K

khanhtoan_qb

Ta có:
[TEX]DM = \frac{1}{2} EF \Rightarrow EF = 5[/TEX]
Đặt DE = x (x > 0)
\Rightarrow [TEX]DN^2 = 16 - x^2 \Rightarrow DF^2 = 64 - 4x^2[/TEX]
Py ta go có:
[TEX]EF^2 = DE^2 + DF^2 = x^2 + 64 - 4x^2 \Rightarrow 25 = 64 - 3x^2 \Rightarrow x = \sqrt{13} \Rightarrow [/TEX]
 DF = ...
 
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