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Index fossil

An index fossil is a fossil that is found in rocks of only one geologic period and is therefore used to assign a date to any rock that it is found in.

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Quadratic formula

... does not depend on x) to the expression to the left of "=", that will make it a perfect square trinomial of the form x2 + 2xy + y2. Since "2xy" in this case is (b/a)x, we ...