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2014-05-19T11:07:34+07:00
A=U1= 5 cm U7=320 cm

U7/U1=a.r pangkat 6
320/5=r pangkat 6
64=r pangkat 6
r=2

S7=a. r pangkat 7 - 1/r - 1
     =5.128-1/2-1
     =635 cm
1 5 1
2014-05-19T11:10:31+07:00
 U_{7} = 5 x  r^{7-1}
320 = 5 x  r^{6}
320 : 5   =  r^{6}
64          =  r^{6}
r             =  \sqrt[6]{64}
r            = 2

 S_{n} = a x  r^{n} - 1 / r-1
S_{7} = 5 x   2^{7} -1 / 2-1
 S_{7} = 5 x 128 - 1
 S_{7} = 640 - 1
 S_{7} = 639


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