Question

Tolong yang tau cara nomer 12 bantu saya... Besok harus dikumpulkan... Terima Kasih

Answer 1

S5 = 55
S3 = 21

a)
Sn = n/2 (2a + (n-1)b)
S5 = 5/2 (2a + (5-1)b)
55 = 2,5 (2a + 4b)
55 = 5a + 10b

S3 = 3/2 (2a + (3-1)b)
21 = 1,5 (2a + 2b)
21 = 3a + 3b

55 = 5a + 10b
dikali 3
165 = 15a + 30b

21 = 3a + 3b
dikali 5
105 = 15a + 15b

165 = 15a + 30b
105 = 15a + 15b
dikurang

60 = 15b
60/15 = b
4 = b

21 = 3a + 3b
21 = 3a + 3(4)
21 = 3a + 12
21-12 = 3a
9 = 3a
9/3 = a
3 = a

b)
Un = a + (n-1)b
U8 = 3 + (8-1)4
U8 = 3 + (7)4
U8 = 3 + 28
U8 = 31