tentukan jumlah 20 suku pertama deret aritmatika berikut! 1.000+996+992+988..

a = 1000
b = -4
Un = 1000 + (n-1)-4
      = -4n + 1004

u20 = -20 + 1004 = 984

S20 = 20/2 (1000 + 984)
       = 10(1984)
       = 19.840

a= 1000
b = -4
U20 = a + 19b
         = 1000 + 19(-4)
         = 1000 - 76
         = 924
S20 = 1/2 . n ( a + U20)
         = 20/2 (1000 + 924)
         = 10 (1924)
         = 19240