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How could you factorise $\displaystyle (-1)^{k-1} + (-1)^{k}$

Quote: Originally Posted by Ifailatmaths How could you factorise $\displaystyle (-1)^{k-1} + (-1)^{k}$ The common factor is $\displaystyle (-1)^{k-1}$. Note: $\displaystyle (-1)^k = (-1)^{k-1} (-1)$.