Ta co: OA = 2.OF = 24cm
xét \(\Delta ABO\infty\Delta A'B'O\left(gg\right)\)
\(\Rightarrow\dfrac{AB}{A'B'}=\dfrac{OA}{OA'}\Rightarrow A'O=\dfrac{A'B'.OA}{AB}=\dfrac{24.A'B'}{5}\)
xét \(\Delta OF'I\infty\Delta A'F'B'\left(gg\right)\)
\(\Rightarrow\dfrac{OF'}{A'F'}=\dfrac{AB}{A'B'}\Leftrightarrow\dfrac{OF'}{OA'-OF'}=\dfrac{AB}{A'B'}\)
Ta co: \(\dfrac{OF'}{\dfrac{24.A'B'}{5}-OF'}=\dfrac{AB}{A'B'}\)
\(\Leftrightarrow\dfrac{OF'.5}{24A'B'-5.OF'}=\dfrac{5}{A'B'}\)
\(\Leftrightarrow\dfrac{60}{24.A'B'-60}=\dfrac{5}{A'B'}\)
\(\Leftrightarrow60A'B'=120.AB'-300\)
\(\Leftrightarrow A'B'=5\)
=> OA'= 24cm
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