Suatu barisan aritmatika U3 = 9 dan U8 = 4, tentukan a (suku pertama) dan beda (b) U=100. Tolong jawab menggunakan caranya....

U3 = a + 2b = 9
U8 = a + 7b = 4
------------------------- -
               -5b = 5
                  b = -1

a + 2b = 9
a + 2(-1) = 9
a - 2 = 9
a = 9 + 2
a = 11

a) suku pertama = 11
     beda = -1

b) U100 = 11 + (100 - 1) (-1)
              = 11 + (99 x (-1))
              = 11 + (-99)
              = -88