Question

determine a soma dos termos da P.A (14, 21, 28,.....,217)

Answer 1

217=14+(n-1).7
217=14+7n-7
217-7=7n
7n=210
n=210/7
n=30 termos

soma dos termos de uma pa:

Sn=(A1+an).n/2

Sn=(14+217).30/2
Sn=231.15
Sn=3465

Answer 2

a1 = 14

an = 217

R = 21-14 = 7



an = a1 + ( n - 1 ).R

217 = 14 + (n-1).7

7n - 7 + 14 =  217 ==> 7n = 217 - 7

7n = 210 ==> n = 30
===========================================

Sn = ( a1+an).n
               2

S30 = ( 14+ 217).30
                  2

S30 = 231.15

S30 = 3.465