Suatu barisan aritmatika diketahui suku ke-5 = 32 dan suku ke-10 = 12.

Jawaban:

U5 = 32

U10 = 12

A.

Mencari b

= 12-32/10-5

= -20/5

= -4

b = -4

Mencari U1 melalui U5

U5 = a + ( 5-1) -4

= 32 = a + (-16)

= a = 48

Maka U2

= U1 + b

= 48 + (-4)

= 44

B.

U25 = a + (n-1)b

U25 = 48 + (25-1)-4

U25 = 48 + 24 . (-4)

U25 = 48 + (-96)

U25 = -48

C.

S25 = n/2 (2a+(n-1)b) atau n/2 ( a + un)

S25 = 25/2 ( 48 + U25 )

S25 = 25/2 ( 48 + (-48))

S25 = 25/2 . 0

S25 = 0