Answer:
No solution
Explanation:
We have

For the sum it is not correct to assume

Note that for

it is assumed
and in your case
for

In fact, considering a set
we have
that satisfy

This means that, by definition

Therefore,

because the sum is empty.
For

we have other problems. Actually, this case is really bad.
Note that
has no value. In fact, if we consider for the case
, the cosine function oscillates between
, and therefore it is undefined. Thus, we cannot evaluate

and then

has no solution