Question

а3=6 d=1,2 S7=? арифметическая прогрессия

Answer 1

a3=a1+1.2*(3-1)
6=a1+2.4
a1=6-2.4
a1=3.6
Sn= (2*a1+d(n-1)/2)*n
S7=(2*3.6+1.2*(7-1)/2)*7=50.4

Answer 2

спасибо)

Answer 3

S7=(a1+a7)*7/2
a1=a3-2d=6-2.4=3.6
a7=a1+d(7-1)=3.6+1.2*6=3.6+7.2=10.8
S7=(3.6+10.8)*7/2=100.8/2=50.4