# simple optics: find focus!!

Level pending

A marginal ray from infinity strikes a concave mirror at a distance of 'b' from the principal axis of the mirror. it is reflected from point Q and intersects the principal axis at point P. O is the optical center of the lens. find distance OP i.e. 'f'. if 'f' can be given by : $f=R(p/q-r/\sqrt{s})$ find $p+q+r+s$ Given p, q, r and s are integers. and $b=R/2$

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