pada barisan arietmatika diketahui U4=15 dan u12=39. suku ke 15 adalah

U4 = a+3b = 15
U12 = a+11b = 39

a+3b = 15
a+11b = 39
________ -
-8b = -24
b = 3

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

U15 = a+14b
= 6 + 14(3)
= 6 + 42
= 48

15+(15-1)39
15+14*39
15+546
561