Which is equivalent to 5th root of 1 comma 215 to the x th power ?

A fifth root is the same as raising a quantity to the power of 1/5. So the fifth root of 1,215^x is (1,215^x)^(1/5) = 1,215^(x/5), which matches 1,215 raised to the 1/5 x power. It is not 243^x, because 1,215 is not equal to 243.

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