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\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\\\ u = -f\\\\ \frac{1}{v}+\frac{1}{f}=\frac{1}{f}\\\\ \frac{1}{v}=0,\ \ v = \infty \\\\ Hence\ the\ image\ is\ formed\ at\ infinite\ distance..\\

When the object is placed at the focus of a lens or a mirror, the image is always formed at the infinity.. It means there is no image formed.  The rays from any point on the object become parallel rays.
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The image will form at infinity . 
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