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KöMaL 2021 C1667

    Let A=(−1)1+(−1)2+(−1)3+⋯+(−1)2021,B=(−2)1+(−2)2+(−2)3+⋯+(−2)2021\begin{align*} A & ={(-1)}^1+{(-1)}^2+{(-1)}^3+\dots+{(-1)}^{2021},\\ B & ={(-2)}^1+{(-2)}^2+{(-2)}^3+\dots+{(-2)}^{2021} \end{align*} and C=(−3)1+(−3)2+(−3)3+⋯+(−3)2021.C={(-3)}^1+{(-3)}^2+{(-3)}^3+\dots+{(-3)}^{2021}. Determine the last digit of the number B+C−AB+C-A.

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