a_1=4
a_5=\frac{64}{81}
a_1*q^4=a_5
4*q^4=\frac{64}{81}/:4
q^4=\frac{\not64^{16}}{81}*\frac{1}{\not4^1}
q^4=\frac{16}{81}
q^4=(\frac{2}{3})^4
q=\frac{2}{3}
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x=a_2=a_1*q=4*\frac{2}{3}=\frac{8}{3}
y=a_3=a_2*q=\frac{8}{3}*\frac{2}{3}=\frac{16}{9}
z=a_4=a_3*q=\frac{16}{9}*\frac{2}{3}=\frac{32}{27}
spr:
a_5=a_4*q=\frac{32}{27}*\frac{2}{3}=\frac{64}{81}
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S_n=a_1*\frac{1-q^n}{1-q}
S_5=4*\frac{1-(\frac{2}{3})^5}{1-\frac{2}{3}}=\frac{4(1-\frac{32}{243})}{\frac{1}{3}}=\frac{4*\frac{211}{243}}{\frac{1}{3}}=\frac{844}{\not243^{81}}*\frac{\not3^1}{1}=\frac{844}{81}=10\frac{34}{81}
Można dodać 5 wyrazów, wspólny mianownik 81.