jumlah 4 suku dan 5 suku pertama barisan aritmatika masing masing 24 dan 35 . suku ke 15 adalah

S5 = 35
S4 = 24
______ -
S5 - S4 = 11
U5 = 11
a + 4b = 11 ... (i)

S4 = 24
4/2 (2a + (4-1)b) = 24
2a + 3b = 12 ... (ii)

Eliminasi pers. (i) dan pers. (ii)
2a + 8b = 22 ... (i)
2a + 3b = 12 .... (ii)
_____________ -
5b = 10
b = 2

U5 = a + 4b
11 = a + 4(2)
a = 11-8
a = 3


U15 = a + 14b
= 3 + 14(2)
= 3 + 28
= 31

s4 = 1/2(4) [2a+ (4-1)b]
24 = 2[2a+3b]
24 = 4a+6b
s5 = 1/2(5) [2a+(5-1)b]
s5 = 5/2 [2a + 4b ]
35= 5a + 10b
dapat 2 persamaan
4a+6b = 24 Dikali 5
5a + 10b = 35 Dikali 4 hasil
20a+ 30b = 120
20a + 40b = 140 dikurangkan
-10b = -20
b = -20/-10 = 2
s4 = 1/2(4) [2a+ (4-1)b]
24 = 2 [2a + 3(2) ]
24 = 4a+12
4a = 24 - 12
a = 12/4 = 3

u 15 = a+14b
u15 = 3+14(2)
u15 = 3+28 = 31