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Penyelesaian:
S₁₁ =11/2 ( a + a + 10b)
110 = 11/2 ( 2a + 10b)
110 = 11/2 × (2) × (a + 5b)
110 = 11 × (a + 5b)
5b = 110/11 - 30
5b = 10 - 30
5b = -20 → b = - 4 → Jawabannya B
Bukti → S₁₁ =11/2 ( 2a + 10b) →S₁₁ = 11/2 ( 2×(30) + 10 × (-4))
= 11/2 (60 + (-40)
= 11/2 ( 20)
= 11 × 10
= 110
12.) Diketahui U₁ = 32 dan Ut = 76, ditanya : Un ?
Penyelesaian : Ut = 1/2 ( U₁ + Un )
76 = 1/2 (32 + Un)
1/2 (32 + Un) = 76
32 + Un = 76/ 1/2
32 + Un = 76 × 2/1
32 + Un = 152
Un = 152 - 32
Un = 120 → Jawabannya C
13.)Diketahui U₅ = 29 dan U₇ = 41. Ditanya : a dan b ?
a + 6b = 41
a + 4b = 29
-------------------- -
2b =12 → b = 12/2 = 6
b = 6 → a + 4(6) = 29
a + 24 = 29
a = 29 - 24
a = 5 → jawabannya A
14.) Diketahui U₄ = 14 dan U₈ = 26. Ditanya : S₃₅ ?
penyelesaian :
a + 7b = 26
a + 3b = 14
---------------- -
4b = 12 → b = 3
b = 3 → a + 3(3) = 14
a + 9 = 14
a = 14 - 9
a = 5
Sn = n/2 ( a + Un)
Sn = n/2 ( a + a + (n - 1)b = 2a + (n - 1)b
S₃₅ = 35/2 {2 (5) + 34 (3)}
= 35/2 ( 10 + 102 )
= 35/2 ( 112)
= 35 × 56
= 1.960 → Jawabannya B
15.) Diketahui : a = 20, b = 4. Ditanya : U₁₅ = ?
Penyelesaian :
U₁₅ = a + 14b
= 20 + 14(4)
= 20 + 56
= 76 → Jawabannya C