Question

Jumlah 30 suku pertama dari barisan 28,25,22,19,.... adalah...

Answer 1

28,25,22,19,.... 

a = 28
b = -3

s30 = (30/2)(2.28 + (30-1).(-3))
       = 15.(56 - 87)
       = 15.(-31)
       = -465

Answer 2

A = U1 = 28

b = U2 - U1
b = 25 - 28
b = -3

S30 = n/2 . (2a + (n - 1) . b)
S30 = 30/2 . (2 . 28 + (30 - 1) . (-3))
S30 = 15 . (56 + 29 . (-3))
S30 = 15 . (56 - 87)
S30 = 15 . (-31)
S30 = -465