Question

Sa se gaseasca primul termen si ratia unei progresii aritmetice
S10=8•Și
S3=-3

Answer 1

s10=8 s5
stim ca sn=(a1+an)*n/2
an=a1+(n-1)r
(a1+a1+9r)*5=8(a1+a1+4r)*5/2
(2a1+9r)*5=20(2a1+4r)
10a1+45r=40a1+80r
30a1+35r=0

s3=-3
(a1+a1+2r)*3/2=2(a1+r)*3/2=3a1+3r
3(a1+r)=-3
a1+r=-1
r=-1-a1
30a1+35r=0
30a1-35(1+a1)=0
-35+30a1-35a1=0
-35=5a1
a1=-7
r=-1-a1=-1+7=6