Question

Dik:deretaritmatika U1+U3+U5=30
dan U2+U4+U6=39
TENTUKAN :-SUKU PERTAMA
-BEDA
-JUMLAH 15 SUKU
PERTAMA

Answer 1

Verified answer

Bab Barisan dan Deret
Matematika SMP Kelas IX

U1 + U3 + U5 = 30
a + a + 2b + a + 4b = 30
3a + 6b = 30
3 (a + 2b) = 30
a + 2b = 30/3
a + 2b = 10    → U3

U2 + U4 + U6 = 39
a + b + a + 3b + a + 5b = 39
3a + 9b = 39
3 (a + 3b) = 39
a + 3b = 39/3
a + 3b = 13 → U4

1] a + 3b = 13
    a + 2b = 10
  ------------------ -
           b = 3

a + 2b = 13
a + 2 . 3 = 13
a = 13 - 6
a = 7

2] jadi, suku pertama = 7 dan beda = 3

3] Sn = n/2 x (2a + (n - 1) x b)
S15 = (15/2) x (2 x 7 + (15 - 1) x 3)
S15 = (15/2) x (14 + 42)
S15 = (15/2) x 56
S15 = 420