Question

Hitunglah jumlah deret aritmetika yang suku pertamanya = 10, suku terakhir = 370, dan U12 - U9 = 72

Answer 1

A = 10

U12 - U9 = 72
(a + 11b)-(a + 8b) = 72
(10+11b)-(10+8b) = 72
3b = 72
b = 72/3
b = 24

sehingga
Un = a+(n-1)b
Un = 10 +(n-1)24
Un = 10 + 24n - 24
Un = -14 + 24n
Un = 24n-14

370 = 24n - 14
370 + 14 = 24n -14 + 14
384 = 24n
n = 384/24
n = 16

nah,
Sn = n/2 (a+Un)
S16 = 16/2 (10 + 370)
S16 = 8(380)
S16 = 3040