Costum trendline

How can I create a trendline of the form y=cte/x, so y=cte*x^(-1)???
Thanks

so not as easy after all...
maybe with the power function, that's something like cte1+cte2*x^cte3, so if you could say that cte3 has to be -1, then it could calculate it just as i want... anybody who can solve this???
regards, RoDeNtJe