The answer is 3.

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Cross multiplying, we get $a^2-2ab+b^2=ab$ .

Simplifying, we have $a^2-3ab+b^2=0$

Dividing by $b^2$ , we get $\left(\dfrac{a}{b}\right)^2-3\cdot\left(\dfrac{a}{b}\right)+1=0$

Applying vietas, we get $\boxed{3}$