Question

Determine a soma das PA:
a)(9+12+15+...213)
b)(2+7+12+...+257)

Answer 1

Resposta:

a)

a1=9

razão=12-9=3

an=a1+(n-1)*r

213=9+(n-1)*3

n=69

Sn=(a1+an)*n/2=(9+213)*69/2 =7659

b)

a1=2

razão=7-2=5

257=2+(n-1)*5

n=52

Sn=(2+257)*52/2 = 6734