Răspuns:
b) 15
Explicație pas cu pas:
[tex]Termenul\ general\ al\ unei\ progresii\ geometrice\ este\ dat\ de\ formula\\\\\b_n=b_1*q^{n-1},\ unde\ q\ este\ ratia\\\\Suma\ a\ n\ termeni\ dintr-o\ progresie\ geometrica\ este\ data\ de formula\\\\\S_n=\frac{b_1*(q^n-1}{q-1}\\\\b_2=b_1*q=2,\ b5=b_1*q^4=16\\\\\frac{b_1*q^4}{b1*q}=q^3=8\ => q=2\ =>b_1=2:2=1 \\\\S_4=\frac{1*(2^4-1)}{2-1}=16-1=15[/tex]
