Diketahui suatu deret aritmatika U5=15 dan U14=51 hitunglah jumlah 19 suku pertama?? Di jawab gan

a + 4b = 15
a + 13b = 51
------------------- (-)
-9b = -36
b = 4

a + 4b = 15
a + 4(4) = 15
a + 16 = 15
a = 15-16
a = -1

S19 = n/2 (2a + (n-1)b)
= 19/2 (2(-1) + (19-2)4)
= 9,5 x (-2)+72
= 9,5 x 70
= 665