di ketahi barisan arimatika U3 =9 dan U8 = -6 suku ke -15 adalah

U3 = a + 2b = 9
U8 = a + 7b = -6
---------------------- -
-5b = 15
b = 15/-5
b = -3

a + 2b = 9
a + 2.-3 = 9
a - 6 = 9
a = 6 + 9
a = 15

U15 = a + 14b
U15 = 15 + 14.-3
U15 = 15 - 42
U15 = -27

semoga membantu

U3=9
a+2b=9....(1)

U8=-6
a+7b=-6..... (2)

a+2b=9
a+7b=-6
________ -
-5b=15
b= -3

a+2b=9
a+2(-3)=9
a-6=9
a=9+6
a=15

U15= 15+14(-3)
= 15 - 42
= -27