tolong bantu jawab please... soalnya hari ini di kumpulkan,makasih

Sn = n/2 (2a + (n-1) b)

S5 = 5/2 (2a + ( 5-1) b)
55 = 5/2 (2a + 4b)
55 = 5a + 10b ...... : 5
11 = a + 2b ....... persamaan 1

S9 = 9/2 ( 2a + (9-1) b)
171 = 9/2 (2a + 8b)
171 = 9a + 36b ...... : 9
19 = a + 4b ........ persamaan 2

Eliminasi Persamaan 1 terhadap 2
11 = a + 2b
19 = a + 4b
—————— -
-8 = -2b
b = 4

substitusi b ke persamaan 1
11 = a + 2(4)
11 = a + 8
a = 3

A. Rumus Sn :
=> Sn = n/2 (2a + (n-1)b)
=> Sn = n/2 (2.3 + (n-1) 4)
=> Sn = n/2 (6 + 4n - 4)
=> Sn = 3n + 2n² - 2n
=> Sn = 2n² + n

B. U11 = a + (n-1) b
=> U11 = 3 + (10) 4
=> U11 = 3 + 40 = 43

C. S12 = 2n² + n
=> S12 = 2(12)² + 12
=> S12 = 2(144) + 12
=> S12 = 288 + 12 = 300

semoga membantu
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