You mean the multiple root theorem?

It's related to my method (from the observation that the triple root

remains a root of the second derivative), but the theorem itself isn't actually being used at all. It's only differentiation by the product rule and equating identities. Both of those are 2U methods, just applied at a higher level.

Also, as a side note I've heard that 4U content is accepted in 3U for full marks (and vice versa for 3U and 2U), but the only caveat is that an error means a zero, since the working out is not "valid". If you prove the content before using it I think it's okay though.

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