diketahui deret aritmetika 18 + 15 +12 + ... jumlah 20 suku pertama deret tersebut adalah

Deret Aritmatika
U1 = a = 18
U2 = a + b = 15

b = U2 – U1
b = 15 – 18
b = –3

Sn = ½n (2a + (n–1)b)

S20 = ½ × 20 (2 × 18 + (20–1)–3)
S20 = 10 (36 – 57)
S20 = 10 × (–21)
S20 = –210

a = 18
b = U2 - U1 = 15 - 18 = -3

Sn = n/2 (2a + (n-1)b)
S20 = 20/2 (2 . 18 + (20-1) (-3))
= 10 (36 + 19 . (-3))
= 10 (36 - 57)
= 10 . (-21)
= -210

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