(а1+an)/2=Sn/n
a1=2Sn/n-an=2*105/7-21=9
an=a1+d(n-1)
d=(an-a1)/(n-1)=(21-9)/6=2
Sau asa
а7=21 a7 = a1+6d = 21 (1)
S7=105 S7 = (2a1+6d)*7/2 = 105 (2)
(1): a1 = 21 - 6d
(2): (a1+3d)*7 = 105
Подставим (1) в (2):
(21-6d + 3d) = 105/7
21-3d = 15
3d = 6
d=2
a1= 21-12=9
Ответ: 9 и 2.
