Question

1 - Determine a soma dos termos da PA
(13, 17, 21, ..., 101).

Answer 1

an = a1+(n—1)*r

101 = 12+(n—1)*4

101 = 12+4n—4

101+3—12 = 4n

92/4 = n

n = 23

Sn = (a1+an)*n/2

Sn = (13+101).n/2

Sn = 114*23/2

Sn = 2622/2

Sn = 1311

Dúvidas??Comente!!

Answer 2

resolução!

r = a2 - a1

r = 17 - 13

r = 4

an = a1 + ( n - 1 ) r

101 = 13 + ( n - 1 ) 4

101 = 13 + 4n - 4

101 = 9 + 4n

101 - 9 = 4n

92 = 4n

n = 92/4

n = 23

Sn = ( a1 + an ) n / 2

Sn = ( 13 + 101 ) 23 / 2

Sn = 114 * 23 / 2

Sn = 57 * 23

Sn = 1311