[tex]7^1=7\\
7^2=49\\
7^3=343\\
7^4=2401\\
u(7^{4k+1})=1\\
u(7^{4k+2})=9\\
u(7^{4k+3})=3\\u(7^{4k})=7\\[/tex]
Ultima cifra a sumei primilor 4 termeni consecutivi este 0.
Ultima cifra a primilor 40 de termeni consecutivi este de asemenea 0.
Ultima cifra a sumei date este
[tex]u(u(7^{41})+u(7^{42})+u(7^{43}))=u(7+9+3)=9\\[/tex]
