suatu deret aritmstiks suku ke 8=18 dan suku ke 12=34 nilai suku ke 18 adalah

U8=18. <=> a+7b=18
U12=34<=>a+11b=34
eliminasi ________ -
menjadi<=>-4b= -16 <=> b= -16/-4= 4
disubstitusi ke persamaan a+7b=18
<=> a+7(4)=18
<=> a+28=18
<=> a=18-28
<=> a= -10
mencari U18
<=>a+17b
<=>(-10)+17(4)
<=>(-10)+68
<=> 58

U12= a+11b =34
U8 = a+7b =18
____________-
4b = 16
b = 16/4
b= 4

a+7b=18
a+7(4)=18
a+28=18
a= 18- 28
a= -10

U18 = a+(n-1)b
= -10 + (18-1)4
= -10 + (17) 4
= -10 + 68
= 58