First of all, you need to understand why Taylor formula is used to expand, in order to get the third order and all the items below the third order infinitesimal order. If all the first ones are multiplied by 2x, Taylor's formula is used to expand to at least the second-order infinitesimal, so that the second-order infinitesimal multiplied by an x is the third-order infinitesimal, and the third one itself has 8x 3, so it only needs to be expanded to the first-order infinitesimal, because if it is expanded to (1+x+o(x)) and 8x 3, it is 8x 3+o (. . .

You got it? The main thing is that you only need to go to the high-order infinitesimal of X 3 to simplify it.

Good luck in postgraduate entrance examination.

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