[tex]\displaystyle\\ \bf\\ a=0,5\times1+0,5\times3+0,5\times5+\cdots+0,5\times1999\\
~~~~~~~~_\texttt{\bf Dam factor comun pe 0,5.}\\
a=0,5(1+3+5+\cdots+1999)\\
~~~~~~~~_\texttt{\bf Calculam numarul de termeni ai sirului.}\\\\
n = \frac{1999-1}{2}+1= \frac{1998}{2}+1=999+1=1000 ~\texttt{ de termeni}\\\\
a = 0,5 \times\frac{1000(1999+1)}{2} =0,5\times \frac{1000\times2000}{2}=\\\\
=0,5\times 1000\times 1000 =500 \times 1000 = \boxed{\bf 500\:000}\in N
[/tex]
