If $\left|\mathrm{r}\right|<1\text{ and }\mathrm{x}=\mathrm{a}+\frac{\mathrm{a}}{\mathrm{r}}+\frac{\mathrm{a}}{{\mathrm{r}}^2}\dots \dots \mathrm{y}=\mathrm{b}-\frac{\mathrm{b}}{\mathrm{r}}+\frac{\mathrm{b}}{{\mathrm{r}}^2}\dots \dots .,\mathrm{z}=\mathrm{c}+\frac{\mathrm{c}}{{\mathrm{r}}^2}+\frac{\mathrm{c}}{{\mathrm{r}}^4}+\dots \dots .\text{then }\frac{\mathrm{xy}}{\mathrm{ab}}=$

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