diberikan barisan 6,9,12,15... suku ke 40 adalah

a= 6
b=u2-u1=9-6=3

U40= a + 39b = 6 + 39(3) = 6 + 117 = 123

a = U1 = 6

b = U2 - U1
b = 9 - 6
b = 3

U40 = a + (n - 1) . b
U40 = 6 + (40 - 1) . 3
U40 = 6 + 39 . 3
U40 = 6 + 117
U40 = 123