Question

se a soma dos 3 termos de uma P.G. vale 78 e a razão 3, quanto vale a1?​

Answer 1

Resposta:a1=6

Explicação passo a passo:

S3=78,n=3,q=3,a1=?

Sn=a1.[(q^n)-1]/q-1

78=[(3^3)-1].a1/3-1

78=[27-1].a1/2

78=26.a1/2

78=13.a1

a1=78/13

a1=6

Answer 2

$> \: resolucao \\ \\ \geqslant \: progressao \: \: geometrica \\ \\ sn = \frac{a1(q {}^{n} - 1) }{q - 1} \\ \\ 78 = \frac{a1(3 {}^{3} - 1)}{3 - 1} \\ \\ 78 = \frac{a1(27 - 1)}{2} \\ \\ 78 = \frac{a1 \times 26}{2} \\ \\ 156 = 26a1 \\ \\ a1 = \frac{156}{26} \\ \\ a1 = 6 \\ \\ \\ \\ \geqslant \leqslant \geqslant \leqslant \geqslant \leqslant \geqslant \leqslant \geqslant \geqslant \geqslant \geqslant$