Hitunglah jumlah 30 suku pertama dari masing-masing barisan berikut! A.9,11,13,15,... B.150,147,144,141,... C.2,7,12,17,...

B. 150,147,144,141,150

Jawab :

a) 9,11,13,15,...
• dik :
a = 9
b = 11-9 = 2
• dit : S30
• jwb :
Sn = n/2 (2a + (n-1)b)
S30 = 30/2 (2(9) + (30-1)2)
S30 = 15 (18 + (29)2)
S30 = 15 (18 + 58)
S30 = 15 (76)
S30 = 1140

b) 150,147,144,141,...
• dik :
a = 150
b = 147-150 = -3
• dit : S30
• jwb :
Sn = n/2 (2a+(n-1)b)
S30 = 30/2 (2(150)+(30-1)(-3))
S30 = 15 (300 + (29)(-3))
S30 = 15 (300 - 87)
S30 = 15 (213)
S30 = 3195

c) 2,7,12,17,...
• dik :
a = 7
b = 7-2 = 5
• dit : S30
• jwb :
Sn = n/2 (2a+(n-1)b)
S30 = 30/2 (2(7)+(30-1)5)
S30 = 15 (14 + (29)5)
S30 = 15 (14 + 145)
S30 = 15 (159)
S30 = 2385

dikoreksi lagi ya, maaf kalau misal ada salah, semoga membantuu