Jawabanmu

2014-03-11T22:28:11+07:00
Ar = 3/2
r=3/2a
arpangkat4 = 9/128

S₈ ??

ar⁴ = 9/128
a(3/2a)⁴ = 9/128
a(81/16a⁴) =9/128
81/16a³ = 9/128
10368=144a³
a³=72
a=∛72

ar=3/2
r∛72=3/2
2r=3∛72
r=3/2 ∛72

tinggal dimasukkan ke ru,us S₈ nya
kalo bisa jadikan yg terbaik
1 5 1
2014-03-11T22:30:32+07:00
U_2 = \frac{3}{2}
\\U_5 = \frac{9}{128}

\\Maka:
\\U_2 = ar = \frac{3}{2}
\\U_5 = ar^4 = \frac{9}{128}
\\U_5 = arr^3 = \frac{9}{128}
\\U_5 = \frac{3}{2}r^3 = \frac{9}{128}
\\r^3 = \frac{\frac{9}{128}}{\frac{3}{2}}
\\r^3 = \frac{9}{128}.\frac{2}{3}
\\r^3 = \frac{3}{64}
\\r =  \sqrt[3]{\frac{3}{64}} 
\\r =  \frac{\sqrt[3]}{4}

Maka:
U_2 = ar = \frac{3}{2}
\\a = \frac{\frac{3}{2}}{r}
\\a = \frac{3}{2}.\frac{4}{\sqrt[3]{3}}
\\a = \frac{6}{\sqrt[3]{3}}

Sehingga:
\\S_6 = \frac{a(1-r^n)}{1-r} \\S_6 = \frac{\frac{6}{\sqrt[3]{3}}(1-(\frac{\sqrt[3]{3}}{4})^6)}{1-\frac{\sqrt[3]{3}}{4}}

Untuk angkanya, selesaikan sendiri saja. Ga muat di sini

Hasil desimal = 6,4916836366082622961384183909735 dibulatkan jadi 6,49