suatu barisan aritmetika dik U1+U3+U5=30danU2+U4+U6=39besar suku ke-50barisan tersebut adalah

Un = a + (n-1)b

U1 + U3 + U5 = 30
a + a + 2b + a + 4b = 30
3a + 6b = 30
a + 2b = 15

U2 + U4 + U6 = 39
a + b + a + 3b + a + 5b = 39
3a + 9b = 39
a + 3b = 13

Eliminasikan
a + 2b = 15
a + 3b = 13
----------------- -
-b = -2
b = 2

a + 2b = 15
a + 2(2) = 15
a + 4 = 15
a = 15 - 4
a = 11

U50 = a + (50 - 1)b
U50 = 11 + (49 x 2)
U50 = 11 + 98
U50 = 109