Deret geometri 1. 250+50+20+2 Jumlah 7 suku pertama barisan diatas 2)a. 2+6+18+54.....+1458 b.192+96.... Question from @Alfaaarizy

whongaliem 2)a. 2+6+18+54.....+1458 ----> a = 2 : r = 3 ; Un 1458
Un = a . $r^{n - 1}$
1458 = 2 . $3^{n - 1}$
   $3^{n - 1}$ = 729
   $3^{n - 1}$ = $3^{6}$
n - 1 = 6
n = 7
Sn = $\frac{a( r^{n}- 1) }{r - 1}$
S7 = $\frac{2( 3^{7}- 1) }{3 - 1}$
S7 = $\frac{2(2187 - 1)}{2}$
S7 = 2186
b.192+96+48+......+6 ------> a = 192 ; r = 1/2 ; Un = 6
Un = a . $r^{n - 1}$
6 = 192 . $\frac{1}{2} ^{n - 1}$
   $\frac{1}{2} ^{n - 1}$ = $\frac{6}{192}$
   $\frac{1}{2} ^{n - 1}$ = $\frac{1}{2} ^{5}$
n - 1 = 5
n = 6
Sn = $\frac{a(1 - r^{n}) }{1 - r}$
Sn = $\frac{192(1 - \frac{1}{2} ^{6}) }{1 - \frac{1}{2} }$
Sn = $\frac{192( 1 - \frac{1}{64}) }{ \frac{1}{2} }$
Sn = 192 . $\frac{63}{64}$ . 2
Sn = 3 . 63 . 2
Sn = 378
c.3+6+12+24+.....+384 -------> a = 3 ; r = 2 ; Un = 384
Un = a . $r^{n - 1}$
384 = 3 . $2^{n - 1}$
   $2^{n - 1}$ = $\frac{384}{3}$
   $2^{n - 1}$ = 128
   $2^{n - 1}$ = $2^{7}$
n - 1 = 7
n = 8
Sn = $\frac{a( r^{n} - 1) }{r - 1}$
S8 = $\frac{3( 2^{8} - 1) }{2 - 1}$
S8 = $\frac{3(256 - 1)}{1}$
S8 = 3.(255)
S8 = 765
untuk soal no.1 cek kembali karena bukan deret geometri....

DimasWidiantoro dahsyat!

wahyusuherman99 2. kl yg ditanya n;
a. Un = ar^n-1
1458=2.3^n-1
729=3^n-1
3^6 = 3^n-1
n=7
b. Un=ar^n-1
6=192.1/2^n-1
6/192=1/2^n-1
0,3125=1/2^n-1
(1/2)^5=1/2^n-1
n=6
c. Un=ar^n-1
384=3.2^n-1
128=2^n-1
2^7=2^n-1
n=8

DimasWidiantoro top!

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