You know
[tex]i^2=-1[/tex]
Squaring both sides gives
[tex](i^2)^2=(-1)^2\implies i^4=1[/tex]
Then raising both sides to any integer power [tex]n[/tex] gives
[tex](i^4)^n=1^n\implies i^{4n}=1[/tex]
That is, any power of [tex]i[/tex] where the power is a multiple of 4 will always be equal to 1.
So
[tex]i^{20}+1=1+1=2[/tex]
But perhaps you mean to write 20+1 as the exponent, in which case
[tex]i^{20+1}=i^{20}i=i[/tex]
