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Jawab:

a. U1 = 3

b. U6 = -22

c. U26 = -122

d. U30 = -142

e. S4 = -18

f. S6 = -57

Penjelasan dengan langkah-langkah:

Diketahui:
U5 = -17

U25 = -117

Sehingga,

Un = a + (n-1)b

a+4b = -17

a+24b = -117

ditemukan, a = 3 dan b = -5

sehingga, Un = 3 + (n-1)(-5)

Rumus Dasar:

• Un = a + (n-1)b
• Un = U(n-1) + b
• Sn = (a + Un)
• Sn = S(n-1) + Un

a. U1 = a = 3

b. U6

Cara 1

U6 = 3 + 5 × (-5) = 3 -25 = -22

Cara 2

U6 = U5 + b = -17 + (-5) = -22

c. U26

Cara 1:

U26 = 3 + 25 × (-5) = 3 – 125 = -122

Cara 2:

U26 = U25 + b = -117 + (-5) = -122

d. U30

U30 = 3 + 29 × (-5) = 3 – 145 = -142

e. S4

Cara 1:

S4 = 2 (3 + U4)

Cari U4 ⇒ U4 = 3 + 3 × (-5) = -12

S4 = 2 (3 -12) = 2 × (-9) = -18

Cara 2:

Cari S5 terlebih dahulu: S5 = (3 – 17) = -35

Sn = S(n-1) + Un

S5 = S4 + U5

S4 = S5 – U5 = -35 – (-17) = -18

f. S6

Cara 1:

S6 = 3 (3 + U6)

S6 = 3 (3 – 22) = 3 × (-19) = -57

Cara 2:

Sn = S(n-1) + Un

Karena S5 = -35

S6 = S5 + U6 = -35 + (-22) = -57

Semoga membantu! Apresiasi kalian sangat berharga untuk saya 🙂

Feel free to ask if there's something you don't understand from my explanation 🙂