Question

Diketahui deret aritmatika dgn U3 = 11 dan U5 = 31. Jumlah dua puluh suku pertama deret tsb?

Answer 1

Verified answer

DERET ARITMETIKA

Diketahui
U3 = 11
U5 = 31

Beda = (U5 - U3) / (5-3)
Beda = (31 - 11) / 2
Beda = 20 / 2
Beda = 10

Un = a + (n-1)b
U3 = a + (3-1)10
11 = a + 20
11 - 20 = a
-9 = a

Sn = n/2 (2a + (n-1)b )
S20 = 20/2 ( 2(-9) + (20-1)10 )
S20 = 10 (-18 + 190)
S20 = 10 (172)
S20 = 1720

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maaf kalau salah :(