Question

Suatu barisan aritmatika suku ke 2 adalah 19 dan suku ke 5 adalah 31. jumlah 24 suku pertama adalah?
a. 1284
b. 1464
c. 2568
d. 2928
ada cara cepetnya ga?

Answer 1

B = 31-19 / 5-2 = 12/3 = 4
U2 = a+b
19 = a + 4
19-4 = a
15 = a

S24 = 24/2 (2.a + 23b)
= 12 ( 2.15 + 23.4)
= 12 ( 30+92)
= 12 x 122
= 1464 (b)

Answer 2

B = 31 - 19/5-2 = 4, A = u2 = a + (n-1) × b ) = 19 = a + (2-1) × 4 = 19 = a + 4 = a = 19 - 4 = 15 Sn = n/2 ( 2.a + (n-1) . b = 24/2 (2 . 15 + (24-1) . 4 = 12 (30 + 92 ) = 12 × 122 = 1.464 (b)