ảnh thật => TKHT
\(k=\frac{-d'}{d}=\frac{f}{f-d}=-2\)
\(k_1=\frac{-d_1'}{d_1}=\frac{f}{f-d_1}=-1\)
\(\Rightarrow\frac{f}{f-d}=\frac{2f}{f-d_1}\)
\(\Leftrightarrow f-d_1=2\left(f-d\right)\)
\(\Leftrightarrow f-\left(d+10\right)=2f-2d\)
=> f- d = -10
=> \(\frac{f}{f-d}=-2\Leftrightarrow\frac{f}{-10}=-2\)
=> f = 20 cm