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deret geometri

1 + 4 + 16 ..... (n= 8)

cari rasio terlebih dahulu

r = u2/u1
= 4/1
= 4

a = 1

maka
n = 8

sn = a(r^n - 1)/ (r-1)
= 1(4^8 -1)/(4-1)
= 1(65.536 -1)/3
= 65.535/3
= 21.845


2.
5 + 10 + 20 ..... (n = 6 )


cari rasio jg

r = u2/u1
r = 10/5
r = 2

a = 5

suku ke 6, n = 6

sn = a(r^n -1)/(r-1)
= 5(2^6 -1)/(2-1)
= 5 ( 64 -1)/1
= 5 . 63
= 315


3.
300 + (-150) + 75 ...... (n= 7)

cari rasio

r = u2/u1
r = -150/300
r = -1/2


a = 300

suku ke 7

sn = a(1- r^n)/(1-r)
= 300(1 - (-1/2)^7)/ (1-(-1/2)
= 300 ( 1- (-1/128) / (1 + 1/2)
= 300 (1 + 1/128) / (1+1/2)
= 300 (129/128) / (3/2)
= 38.700/128 / (3/2)
= 38.700/128 x 2/3
= 12.900/64
= 201

4. maaf untuk nomer 4 rasio antara u2/u1 . u3/u2 tidak sama jadi saya bgung karena deret geometri rasio harus sama