Question

Calculati suma primilor 9 termeni a unei progresii aritmetice {An} n≥1 stiind ca a1= 3, a3=7

Answer 1

Răspuns:

an=a1(n-1)r

7=3+6r|-3

4=6r

r=4/6

r=2/3

a9=3+8*2/3

a9=3+16/3

a9=3+5.3333...=8.333333 sau 8.(3)

s9=((3+8.3)*9)/2

s9=11.(3)*9/2=50,85