Bias of the ratio estimator from $\operatorname{Cov}(r,\bar x)=\bar Y-\bar X\,E(r)$

Let $r=\bar y/\bar x$ denote the ratio of the sample means of the variable $y$ and of the auxiliary variable $x$. The sample is a SRSWOR sample of size $n$. Let $f=n/N$ and $\bar X$ be the population mean of the auxiliary variable $x$. Let $R=\bar Y/\bar X$. By considering $\operatorname{Cov}(r,\bar x)$, the bias of $r$, as an estimator of R, is given by: