Diberikan U3 = 21 dan U5 = 17, tentukan U25

Barisan aritmatika

U3 = a + 2b = 21

U5 = a + 4b = 17

Eliminasi :

a + 2b = 21

a + 4b = 17

—————– –

-2b = 4

b = -2

Substitusikan nilai b = -2

U3 = a + 2b = 21

U3 = a + 2(-2) = 21

U3 = a + (-4) = 21

U3 = a – 4 = 21

U3 = a = 21 + 4

a = 25

U25 = a + 24b

U25 = 25 + 24(-2)

U25 = 25 + (-48)

U25 = 25 – 48

U25 = -23