Question

Bantu jawab nomor 21 y...

Answer 1

B = 3

U9 : U7 = 5 : 4
(a + 8b)/(a + 6b) = 5/4
(a + 8 × 3)/(a + 6 × 3) = 5/4
(a + 24)/(a + 18) = 5/4
4(a + 24) = 5(a + 18)
4a + 96 = 5a + 90
96 - 90 = 5a - 4a
a = 6

Un = 63
a + (n - 1) . b = 63
6 + (n - 1) . 3 = 63
6 + 3n - 3 = 63
3n + 3 = 63
3n = 63 - 3
3n = 60
n = 60 : 3
n = 20

S20 = n/2 . (a + Un)
S20 = 20/2 . (6 + 63)
S20 = 10 . 69
S20 = 690

Jawaban: B