tolong ya yg dibuletin yg bener !! makasih

Un= a + (n-1) x d
9   = a + (4 - 1) x d
9   = a + 3d
9 - 3d = a

Sn= n/2 (2a + (n-1) d)
120 = 10 / 2 ( 2 (9 - 3d) + ( 10 - 1) d)
120 = 5 (18 - 6d +9d)
120/5 =18 + 3d
24-18 = 3d
6=3d
d= 2
a= 9-3d
  = 9 - (3x2)
  = 9 - 6
  = 3


U1= 3 dan beda barisan = 2