Jawabanmu

2014-03-23T20:10:13+07:00
1 , 3 , 7 , 14 , 25 , ... (a = 1)
  2 , 4 , 7 , 11, ...       (b = 2)
    2 , 3 , 4 , ...          (c = 2)
 U_{20} = [/tex] a + (n-1)b+(n-1)(n-2) \frac{c}{2}
 U_{20} = 1 + (19)2+(19)(18) \frac{2}{2}
 U_{20} = 1 + 38 + 342
 U_{20} = 381
      a + ( n - 1 ) b = 381
           1 + (19) b = 381
                   19 b = 380
                        b= 20

 S_{n} =  \frac{n}{2}  (2a+ (n-1)b)
S_{15} = \frac{15}{2} (2+ (14)20)
S_{15} = \frac{15}{2} (282)
S_{15} = \frac{4230}{2}
S_{15} = 2115

jadi,  U_{20} = 381 dan S_{15} = 2115