The lens formula is given by $\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$, where $f$ is the focal length, $d_o$ is the object distance, and $d_i$ is the image distance. For a diverging lens, the focal length is negative. So, we have $f = -20 \, \text{cm}$ and
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