Jawabanmu

2014-02-18T15:51:10+07:00
UN=a+(n-1)b
21=a+(21-1)3
21=a+60
a=21-60
 =-39
2014-02-18T16:05:52+07:00
B = 3 ;  S_{3} = 21

Sn =  \frac{n}{2} (2a+(n-1)b)
 S_{3}  \frac{3}{2} (2a+(3-1)3)
21 =  \frac{3}{2} (2a+6)
21 = 3a + 9
12 = 3a
4 = a
a = 4