Question

qual a quantidade de termo de pg cuja razao e 3 eo primeiro termo vale 1 e ho ultimo termo vale 81?

Answer 1

$n=?\\ q=3\\ a_1=1\\ a_n=81\\\\\\ a_n=a_1*q^{n-1}\\\\\\ 81=1*3^{n-1} \to \\\\ \frac{81}{1} =3^{n-1} \to \\\\ 81=3^{n-1} \to \\\\ 3^4=3^{n-1} \to \\\\ 4=n-1\to \\\\4+1=n\to\\\\ n=5\\\\\\\\ PG(1, 3, 9, 27, 81)$