Question

1. sa se determine termenii a1,a2,a3 ai progresiei aritmetice a1,a2,a3,11,15,19,...

2. sa se determine suma primilor n termeni ai progresiei aritmetice (an) daca se cunosc:
a1= -5, r=4, n=25
a1=200, r=-2, n=80

Answer 1

1.r=4
a4=a1+(n-1)r     n=4
a4=a1+3r
a1=a4-3r
a1=11-12
a1=-1
asa se fac si restul
a2=3
a3=8
2.a.an=a1+(n-1)5=-5+24*2=91
Sn=(a1+an)*n/2
$Sn= \frac{(-5+91)25}{2}= \frac{96*25}{2} =1200$
b.an=a1+(n-1)-2=200-79*2=42
Sn=(a1+an)*n/2
$Sn= \frac{(200+42)80}{2}=960$