Explicație pas cu pas:
[tex]a_n=a_1+(n-1)r[/tex]
[tex] a_{10}=6+9\cdot 3 =33 [/tex]
[tex]S_n = \dfrac{n}{2}\cdot (a_1+a_6)[/tex]
[tex] S_6 = 3\cdot (6+6+5\cdot 3)= 81 [/tex] [tex] \hfill{\boxdot}[/tex]
an=a1+r(n-1)
a10=a1+r(10-1)
a10=a1+9r
a10=6+9*3
a10=6+27
a10=33
S6=(a1+a6)*n/2
a6=a1+r(6-1)
a6=a1+5r
a6=6+5*3
a6=21
S6=(6+21)*6/2
S6=27*3
S6=81
