Mohon dibantu yaa, pake caranya.. :)) 1. diketahui deret geometri dengan suku ke2 = 15 dan suku ke 5 = 405 jumlah 6 suku pertama deret itu adalah.... a. 1.280 b. 1820 c. 2.180 d. 2810. 2. rumus suku ke-n dari barisan bilangan 1/2 , 1 , 2 ,4 ,8... adalah.. a. 2n+¹ b. 2n c.2n-¹ d. 2n-²
subebe
1. U5 / U2 = ar^4 / ar 405 / 15 = r³ 27 = r³ r = 3 U2 = ar 15 = a 3 a = 5 S6 = a (r^6 - 1) /(r - 1) = 5 (3^6 -1) / (3 - 1) = 5 (729 - 1) / 2 = 5 (728) / 2 = 1.820 (B)
2. a = 1/2 r = 1/ 1/2 = 2 Un = 1/2 x 2^(n-1) Un = 2^-1 x 2^(n-1) Un = 2^(n-2)
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brigittadelvina
1. u2= 15 dan u5=405 yg ditanya S6=...? pertama cari rasionya U5 = 405 U2 15 ar^4 = 405 ar = 15 r^3 = 27 r = 3 terus cari suku pertamanya ar = 15 a= 15/3 =5 cari S6 = a kali ( r^6 - 1) r-1 = 5 kali ( 3^6 - 1) 3-1 = 5 kali ( 729-1) = 1820 2 2. a= 1/2 r - 2 Un= a kali r^(n-1) = 1/2 kali 2^(n-1) = 2^-1 kali 2^(n-1) Un = 2^(n-2)
405 / 15 = r³
27 = r³
r = 3
U2 = ar
15 = a 3
a = 5
S6 = a (r^6 - 1) /(r - 1)
= 5 (3^6 -1) / (3 - 1)
= 5 (729 - 1) / 2
= 5 (728) / 2
= 1.820 (B)
2. a = 1/2
r = 1/ 1/2 = 2
Un = 1/2 x 2^(n-1)
Un = 2^-1 x 2^(n-1)
Un = 2^(n-2)
pertama cari rasionya
U5 = 405
U2 15
ar^4 = 405
ar = 15
r^3 = 27
r = 3
terus cari suku pertamanya
ar = 15
a= 15/3 =5
cari S6 = a kali ( r^6 - 1)
r-1
= 5 kali ( 3^6 - 1)
3-1
= 5 kali ( 729-1) = 1820
2
2. a= 1/2
r - 2
Un= a kali r^(n-1)
= 1/2 kali 2^(n-1)
= 2^-1 kali 2^(n-1)
Un = 2^(n-2)