Պրոգրեսսիա

a1=-7
d=3
Sn=430
Sn=-14+(n-1)3/2*n
430=-14+3n-3/2*n
860=(-17+3n)*n
860=-17n+3n2
3n2-17n-860=0
D=289+10320=10609
n1=17+103/6=120/6=20
n=20

n=24
a1=1
d=1
S24=2+(24-1)1/2*24=2+23/2*24=25/2*24=12.5*24=300