[R] How to solve difficult equations?

[francogrex]

This below is not solvable with uniroot to find "a":
fn=function(a){
b=(0.7/a)-a
(1/(a+b+1))-0.0025
}
uniroot(fn,c(-500,500))  gives
"Error in uniroot(fn, c(-500, 500)) : f() values at end points not of
opposite sign"

I read R-help posts and someone wrote a function:
http://finzi.psych.upenn.edu/R/Rhelp02a/archive/92407.html
but it is not very precise. Is there any '"standard" function in R that can
solve this? thanks.
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