Jawabanmu

2014-02-07T09:57:36+07:00
Un = a+(n-1)b
138= -2 + (n-1)7
138 = -2 + 7n - 7
138 = 7n -9
138 + 9 = 7n
147 = 7n
147/7 = n
21 = n

jadi nilai n = 21
2014-02-07T12:04:04+07:00
Un=a+(n-1)b
138=-2+(n-1)7
138+2=7n-7
140+7=7n
7n=147
n=21
1 5 1