Do you always multiply by conjugate when simplifying radicals in a denominator

If the denominator is simply another radical, then just multiply by the radical over itself. However, if the denominator is an expression with a radical, such as 2-sqrt5, then you need to multiply by its conjugate over itself, which would be 2+sqrt5 in this case.

Note: The reason you multiply by the radical / conjugate over itself is because you are simply multiplying by 1 (since anything divided by itself is 1). However, you are still able to manipulate the denominator regardless.

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