Question

Dari barisan aritmatika diketahui U5=21,dan U7+U11= 78. hitung besarnya U20 !

Answer 1

U5 = a + 4b
21 = a + 4b
U7 + U11 = a + 6b + a + 10b
78 = 2a + 16b
39 = a + 8b
21 = a + 4b-
18 = 4b
b = 4,5
U5 = a + 4b
21 = a + 4(4,5)
21 = a + 18
a = 3
U20 = a + 19b = 3 + 19(4,5) = 88,5

Answer 2

21 = a + 4b
78 = 2a + 16b
42 = 2a + 8b (persamaan pertama dikali 2)
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36 = 8b
b = 4,5 ; a = 3
u20 = 3 + (19×4,5) = 88,5