diketahui barisan aritmatika suku ke -2 = 46 dan suku ke5 = 34. jumlah 23 suku pertama barisan tersebut adalah...

Un = a + (n-1)b

U2 = 46
a + (2 - 1)b = 46
a + b = 46

U5 = 34
a + (5 - 1)b = 34
a + 4b = 34

a + b = 46
a + 4b = 34
----------------- -
-3b = 12
b = -4

a + b = 46
a - 4 = 46
a = 46 + 4
a = 50

Sn = n/2(2a + (n-1)b)
S23 = 23/2 (2(50) + (23-1)(-4))
S23 = 23/2 (100 + (-88))
S23 = 23/2 x 12
S23 = 23 x 6
S23 = 138