Question 58715
Telephoto lens.  Use the formula 1/F=1/O + 1/I to find the image distance I for an object that is 2,000,000 mm from 250 mm telephoto lens.
F=250 mm
0=2,000,000
It's often easier to solve the formula for the value you want and then plug in your values.
{{{1/F=1/O+1/I}}}
{{{1/F-1/O=-1/O+1/O+1/I}}}
{{{1/F-1/O=1/I}}}
{{{O/FO-F/FO=1/I}}}
{{{(O-F)/FO=1/I}}}
{{{((O-F)/FO)^(-1)=(1/I)^-1}}}
{{{FO/(O-F)=I/1}}}
{{{I=FO/(O-F)}}}
Plug in your values:
{{{I=(250)(2000000)/(2000000-250)}}}
{{{I=(500000000/1999750)}}}
{{{I=250.0312539}}}
I=250 mm after you round off to sig. figs.
:

Now, if you think about it, your ariginal formula was:
{{{1/F=1/O+1/I}}}  2,000,000 is aproaching infinity, so 1/2,000,000 is very close to 0.
{{{1/F=0+1/I}}}
{{{F=I}}}  which is what we ended up with.
Happy Calculating!!!