diketahui deret aritmatika suku ke 8 ＝47 suku ke 11 ＝52 tentukan jumlah 20 suku pertama

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

U8 = a + 7b = 47
U11 = a + 10b= 52
---------------------------- -
-3b = -5
b = 5/3 = 1 2/3

a + 10. 5/3 = 52
a + 50/3 = 52
a + 50/3 = 156/3

a = 106/3 = 35 1/3


S20 = 20/2 × ( 2. 106/3 + ( 20 - 1 ) . 5/3 )
= 10 × ( 212/3 + 95/3 )
= 10 × ( 307/3 )
= 3070/3 = 1023 1/3


ini jawabannya
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