Jawabanmu

2014-03-27T15:07:36+07:00
Sn = (a+Un)x n :2
168 = (a+15) x 7:2
168 = (7a+105 ):2
168x2 = 7a + 105
336 = 7a + 105
231 = 7a
a = 231/7
a= 33
^_^

2 3 2
2014-03-27T15:18:00+07:00
Sn = (a+Un). n :2
168 = (a+15) x 7:2
168 = (7a+105 ):2
168.2 = 7a + 105
336 = 7a + 105
231 = 7a
a = 231/7
a= 33
1 5 1