diketahui deret aritmatika suku ke-6= 13 dan suku ke-9= 5 hitunglah jumlah 20 suku pertama?

Barder


U9- U6 = 5- 13
a +8b - (a + 5b) = -8
3b = -8
b = -8/3

a + 5b = 13
a - 40/3 = 39/3
a = 79/3


S20
= 20/2 [ 2(79/3) + 19(-8/3)]
= 10 [ 158-152 /3]
= 10 (2)
= 20