The way to show results like this is to use the parallelogram identity. You can choose almost any two elements of the space to see that they do not satisfy the identity. The easiest choice would be to take for example x to be the sequence in having a 1 for its first coordinate and 0 for every other coordinate; and take y to be the sequence having a 1 for its second coordinate and 0 for every other coordinate. Then x, y, x+y and x–y all have -norm 1