Question

considere a p.a.(10,20,30,.....,200)calcule a soma de todos os seus 20 termos.

Answer 1

PA: a1 = 10
      an = 200
      r= 10
      n = ?
an = a1+(n-1)r
200=10+(n-1)10
200=10+10n - 10 
200= 10n
n= 200/10
n=20


sn = (an+a1)n/2
sn = (200+10)20/2
sn = 210*10
sn = 2100