tentukan jumlah 7 suku pertama pada deret geometri ini a. 2+6+18+54+…….. b. 1+3+9+27+ …… c. 128+64+32+…… d. 9+3+1+1/3+…… e. 18+6+2+2/3+…….

A. 2+6+18+54+……..
a = 2
r = 6/2 = 3
S7 = 2 x (3^7 – 1)/(3-1) = 2 x (2.187 – 1)/2 = 2.186

b. 1+3+9+27+ ……
a = 1
r = 3/1 = 3
S7 = 1 x (3^7 – 1)/(3-1) = 2.186/2 = 1.093

c. 128+64+32+……
a = 128 = 2^7
r = 64/128 = ½
S7 = 128 x ( 1 – ½^7)/(1-½)
= 128 x 2 x (128-1)/128
= 2 x 127
= 254

d. 9+3+1+1/3+……
a = 9
r = 3/9 = ⅓
S7 = 9 x ( 1 – ⅓^7)/(1-⅓)
= 9 x 3/2 x (2.187-1)/2.187
= 27/2.187 x 2.186/2
= 1.093/81
= 13 40/81

e. 18+6+2+2/3+…….
a = 18
r = 6/18 = ⅓
S7 = 18 x (1 – ⅓^7)/(1-⅓)
= 18 x 3/2 x (2.187-1)/2.187
= 27/2.187 x 2.186
= 2.186/81
= 26 80/81