Question

Ada yang bisa nomor 6 sampai7
ternyata susah. coba berapa kali enggak dapat jawabanya.. tolong

Answer 1

 Sn = 3n² - 4n
U₁₁ = S₁₁ - S₁₀
= (3.11² - 4.11) - (3.10² - 4.10)
= 319 - 260
= 59 (B)
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Un = a + (n-1)b
U₈ + U₁₂ = 52
a + 7b + a + 11b = 52
2a + 18b = 52 (semua sisi dibagi 2)
a + 9b = 26 ............(1)

U₁₅ = a + 14b = 11 .......(2)
Gunakakan eliminasi
a + 9b = 26
a + 14b = 11
---------------------------- -
-5b = 15
b = 15/-5 = -3

a + 9b = 26
a + 9.-3 = 26
a = 26 + 27 = 53

Sn =  n/2 (2a + (n-1)b)
S₂₀ = 20/2 (2.53 + (20-1).-3)
     = 10 (106 - 57)
     = 10 x 49
    = 490 (B)