Question

a n арифметическая прогрессия а2=9, а3=18 Найдите s6

Answer 1

a2=9,a3=18,s6=?
d=a2-a1=a3-a2=a4-a3=......
d=18-9=9,d=9
a1=a2-d,a1=9-9=0,a1=0
a6=a1+5d,a6=0+6.9=0+54=54,a6=54
sn=n(a1+an)/2
s6=6(a1+a6)/2
s6=6(0+54)/2=3.54=162
s6=162
=======