Question

Care e ultima cifra a nr 1+1*2+ 1*2*3+ 1*2*3*4 +......1*2*3*4*....*2006?

Answer 1

S=1+1•2+1•2•3+1•2•3•4+…..+1•2•3•4•….•2006

incepand de la 1•2•3•4•5 toate produsele au ultima cifra 0;  (avem 2•5)

deci, u (S)=u(1+1•2+1•2•3+1•2•3•4)=u(33)=3