Question

Diketahui deret aritmatika dgn U3= -1 dan U7= 15, jumlah 8 suku pertama deret tsb adalah.....

Answer 1

U3 = -1
a + 2b = -1

U7 = 15
a + 6b = 15

eliminasi 

a + 6b = 15
a + 2b = -1
_________ -
       4b = 16
         b = 4

a + 2b = -1
a + 2(4) = -1
a + 8 = -1
a = -1 -8
a = -9

S8 = 8/2 ( 2(-9) + (9-1)4)
     = 4 (-18 + 32)
     = 4(14)
     = 56

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