Question

Sa se calculeze suma primilor 5 termeni a unei progresi aritmetice stiind ca a1 =1 si a2=3

Answer 1

Sn=(a1+an)n/2
S5=(a1+a5)x5/2
an=a1+(n-1)r
r=a2-a1=3-1=2
a5=1+4x2=9
S5=(1+9)x5/2=10x5/2=50/2=25

Answer 2

[tex]a_2 = a_1 + r =\ \textgreater \ r = a_2-a_1 =3-1=2 \\ a_5 = a_1+4r = 1+8 = 9\\ S_5 = \frac{(a_1+a_5) \cdot5 }{2} = \frac{10 \cdot 5}{2} =25[/tex]