Question

Numa PG de razão 2, a5 = 64, determine a1.

Answer 1

$a_n=a_5=64\\ a_1=?\\ n=5\\ r=q=2\\ \\ a_n=a_1.q^{n-1}\\ \\ 64=a_1.2^{5-1}\\ \\ 64=a_1.2^4\\ \\ 64=a_1.16\\ \\ a_1=64\div16\\ \\\fbox{ a_1=4 }$