suku kedua puluh delapan barisan aritmatika 45,38,31,24.....adalah

a = 45
b = 38 - 45 = - 7
Un = a + ( n - 1 ) b
U20 = a + ( 20 - 1 ) b
U20 = a + 19b
= 45 + 19(-7)
= 45 - 133
= - 88