Question

Dari suatu deret geometri diketahui bahwa U6+U9+U12+U15 = 20 maka S20=

Answer 1

Un = a + (n - 1)b

U6 = a + 5b
U9 = a + 8b
U12 = a + 11b
U15 = a + 14b

U6 + U9 + U12 + U16
= 20

(a + 5b) + (a + 8b) + (a + 11b) + (a + 14b) = 20

a + a + a + a + 5b + 8b + 11b + 14b
= 20

4a + 38b = 20
---> 2a + 19b = 10

Sn = n / 2 × 2a + (n - 1)b
S20 = 20/2 × 2a + (20 - 1)b
S20 = 10 × (2a + 19b)
S20 = 10 × 10
S20 = 100.