Jawabanmu

2014-03-19T12:41:44+07:00
  Sn = n² + 5n
S20 = 20² + 5.20
       = 400 + 100
       = 500
2014-03-19T15:35:02+07:00
Sn = n² + 5n
    subsitusikan n = 20
S20 = 20² + 5x20
S20 = 400 + 100
S20 = 500