Question

determine o primeiro termo da pg,tal que a1+a4=28 e a2+a5=84

Answer 1

$a_1+a_4=a_1+a_1.q^3=28\\ a_2+a_5=a_1.q+a_1.q^4=84\\ \\ a_1(1+q^3)=28\\ a_1(q+q^4)=84\\ \\ \frac{q+q^4}{1+q^3}=\frac{84}{28}\\ \\ \frac{q+q^4}{1+q^3}=3\\ \\ \frac{q(1+q^3)}{1+q^3}=3\\ \\ \boxed{q=3}$

$a_1+a_1.3^3=28\\ \\ a_1+27a^1=28\\ \\ \boxed{a_1=1}$