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I know that $ 2 \sqrt{9-x^2}$ is the alternate form for $\sqrt{36 - 4x^2}$. I tried but i didn't figure out how to get there. Can someone help?

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This is a consequence of the fact that $\sqrt{ab}=\sqrt{a}\sqrt{b}$. Note that in your case, we have the following: $$\sqrt{36-4x^2}=\sqrt{4(9-x^2)}$$ Applying what I wrote earlier, we are left with the following. $$\sqrt{4}\sqrt{9-x^2}$$ This of course simplifies to $2\sqrt{9-x^2}$.

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