Question

a)a1=1;an=20;n=50 b)a1=1;an=200;n=100 sn?

Answer 1

1) a1=1 n=50 a50=20
S(50)=((a1+a50)/2)*50=((1+20)/2)*50=(21/2)*50=21*25
S(50)=525


2) S(100)=((1+200)/2)*100=(201/2)*100=201*50
S(100)=10050
вроде бы не ошибаюсь.