What is the rule that you can use to calculate (√x - √y）ˆ2

$$\boxed{(a-b)^2=a^2-2ab+b^2}\\\\
so\\\\
(\sqrt{x}-\sqrt{y})^2\\\\
=(\sqrt{x})^2\;-\;2\sqrt{x}\sqrt{y}\;+\;(\sqrt{y})^2\\\\
=x-2\sqrt{xy}+y\\\\
=x+y-2\sqrt{xy}\\\\$$

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