Question

Tolong, pakai caranya ya

Answer 1

6.
Un = a + (n - 1)b
U3 = a + (3 - 1)b
9 = a + 2b .....(1)

U5 + U7 = a + (5 - 1)b + a +(7 - 1)b
36 = 2a + 4b + 6b
(36 = 2a + 10b)/2
18 = a + 5b .....(2)

eliminasikan (1) dan (2)
9 = a + 2b
18 = a + 5b
_________ -
-9 = -3b
b = -9/-3 = 3

masukkan nilai b ke (1)
9 = a + 2b
9 = a + 2(3)
9 = a + 6
a = 9 - 6 = 3

Sn = n/2 (2a + (n - 1)b)
S15 = 15/2 (2(3) + (15 - 1)(3))
= 15/2 (6 + 14(3))
= 15/2 (6 + 42)
= 15/2 (48)
= 15 × 24 = 360

7.
S4 = 4/2 (2a + (4 - 1)b)
17 = 2(2a + 3b)
17 = 4a + 6b .....(1)

S8 = 8/2 (2a + (8 - 1)b)
[58 = 4(2a + 7b)]/2
29 = 2(2a + 7b)
29 = 4a + 14b .....(2)

eliminasikan (1) dan (2)
17 = 4a + 6b
29 = 4a + 14b
___________ -
-12 = -8b
b = -12/-8 = 3/2

masukkan nilai b ke (1)
17 = 4a + 6b
17 = 4a + 6(3/2)
17 = 4a + 9
17 - 9 = 4a
8 = 4a
a = 8/4 = 2
U1 = a = 2

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