limn→∞ 20∑x=1 cos 2n(x−10) is equal to
0
1
19
20
limn→∞ cos 2nx = {1,x=rπ,r∈I0,x≠rπ,r∈I
Here, for x = 10, limn→∞cos2n(x−10) = 1
and in all other cases it is zero
∴limn→∞20∑x=1cos2n(x−10) = 1