Question

Uma PA tem a1= 1 e r= 1 . Determine a soma dos seus: (A) 10 primeiros termos; (B) 20 Primeiros termos.

Answer 1

(A)
(1+10)10/2=55

(B)
(1+20)20/2=210

Answer 2

Soma de termos:

S_n = (a_1+a_n)*n / 2

a_10 = a_1+9r
a_10 = 1+9 = 10

S_10 = (1+10)*10/2
S_10 = 11*5

S_10 = 55

____________________

a_20 = a_1+19r
a_20 = 1+19
a_20 = 20

S_20 = (a_1+a_20)*20/2
S_20 = (1+20)*20/2
S_20 = 21*10

S_20 = 210