Diketahui Un merupakan suku ke_n suatu deret geometri dengan U2=24 dan U5=3. Jumlah 5 suku pertama deret tersebut adalah..

U2 = 24

U2 = a + (n – 1)b

U2 = a + b = 24

U5 = 3

U5 = a + (n – 1)b

U5 = a + 4b = 3

a + b = 24

a + 4b = 3

-3b = 21

b = 21 / -3

b = -7

a + b = 24

a = 24 – b

a = 24 – (-7)

a = 31

S5 = n/2 (2a + (n – 1)b)

S5 = 5/2 (2(31) + (5-1)(-7))

S5 = 5/2 (62 + (-28))

S5 = 5/2 (34)

S5 = 5 × 17

S5 = 85