[tex]\displaystyle
\text{Se da sirul: 75, 78, 81, 84, ... 297, 300, 303} \\ \\
a)~~~ n = \frac{303 - 75}{3}+1 = \frac{228}{3}+1= 76+1 = \boxed{77~ de~numere} \\ \\
b)~~~ S = 75+78+81+84+ ... +297+ 300+303= \\ \\
= \frac{77(303+75)}{2}= \frac{77 \times 378}{2} = 77 \times 189 = \boxed{14553}[/tex]
