##(-1)^{2n}=1##, so in your example the answer alternates between ##1/2## and ##-1/2##. The answer is right, however, for this example, but I meant the question in a general manner.
What I meant, is that if you have sequences ##(x_n), (y_n), (\overline{x_n}), (\overline{y_n})##, then you can...