Арифметическая прогрессия, где а1=х+1; d=a2–a1=x+5–x–1=4; Sn=3200
2a1+4(n–1)
Sn= --------------- • n = (a1+2(n–1))•n =
2
= (x+1+2n–2)•n
а1+аn x+1+x+157
Sn = --------- • n = -------------- • n =
2 2
2x+158
= ----------- • n = (x+79)•n
2
(x+79)•n=(x+2n–1)•n |:n
x+79=x+2n–1
2n=80
n=40
(x+79)•40=3200
x+79=80
x=1