deret aritmatika U2=12 dan U7+U9=48 tentukan S20
Matematika
tsania42
Pertanyaan
deret aritmatika U2=12 dan U7+U9=48 tentukan S20
1 Jawaban
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1. Jawaban cupid85
U2 = a + b = 12
U7 + U9 = a + 6b + a + 8b = 48
= 2a + 14b = 48
a + b = 12 |×2
2a + 14b = 48
2a + 2b = 24
2a + 14b = 48
--------------------(-)
- 12b = - 24
b = 2
a + b = 12
a + 2 = 12
a = 12 - 2
a = 10
S20 = ½ . 20 (2 . 10 + (20 - 1)2)
= 10 (20 + 38)
= 10 × 58
= 580