21;18;15
d=18-21=-3
a1=18
s20=(a1+a20)/2*20
a20=a1+19*d
a20=18+19*(-3)=18-57=-39
s20=(18-39)/2*20=420/2=210
d=18/15-21=-297/15 = -19.8
найдем 20 член прогрессии:
а20 = а1 + 19*d = 21 + 19*(-19.8)= 21- 376.2= - 355.2
S20 = (a1+a20)*20/2 = (a1+a20)*10 = (21-355.2)*10 = (-334.2)*10= - 3342.
ответ: S20= - 3342
