Question

qual e a soma dos termos da P.A finita (-30, -21,-12, ...,213)

Answer 1

sn=(a1+an)*n/2 an=213 a1=-30 r=9

an=a1+(n-1)*r 213=-30+(n-1)*9

213=30+9n-9

213=9n-21

213+21=9n

234/9=n

26=n

sn=(-30+213)*26/2

sn=183*26/2

sn=4758/2

sn=2379

Answer 2

Encontrara a razão da PA

r= a2 - a1

r = -21 - (31)

r = -21 + 31

r = 9

Encontrar o numero de termo da PA

an = a1 + ( n -1) . r

213 = -30 + ( n -1) . 9

213 = -30 + 9n - 9

213 = -39 + 9n

252 = 9n

n = 28

Sn = ( a1 + an ) . n / 2

Sn = ( -30 + 213 ) . 28 / 2

Sn = 183 . 14

Sn = 2562