What is the units digit of $3^{2^{1000}} + 1$? (The 1000th Fermat prime)
Units digit of \(3^{4k}\) is always 1 so the units digit of this is 2.
Also I think 2^2^1000 +1 is what you mean, if that is it, then the units digit of \(2^{4k}\) is always 6 so the units digit of this is 7
+178 
