Question

Numa P.A o 7 termo vale 6 e o 20 termo vale 149 . Determine a soma dos seus 25 termos iniciais .

Answer 1

Sn=(a1+an).n/2   

S25=(-60+204).25/2

S25=144.25/2

S25=72.25

S25=1800

an=a1+(n-1).r

an=-60+(25-1).11

an=-60+24.11

an=204

a20=a7+6r

149=6+13r

13r=143

r=143/13

r=11

a7=a1+6r

a1+6.11=6

a1=6-66

a1=-60