Three "touchy" circles

Geometry Level 4

Three circles with radii $$r_1,r_2,r_3$$ (where $$r_1<r_2<r_3$$ ) touch each other externally.If they have a common tangent,the value of $\sqrt{\frac{r_1}{r_2}} +\sqrt{\frac{r_1}{r_3}}$ is?

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