DERET ARITMATIKA DAN GEOMETRI NO 4-5

4. U11 = a+10b = 42
U 5 = a+4b = 18 -
--------------------
6b = 24
b = 4
a+10b = 42
a+10(4) = 42
a+40 = 42
a = 42-40
a = 2

Sn = n/2 (2a+(n-1)b)
S30 = 30/2 (2(2) + 29(4)
= 15 (4+ 116)
= 15 (120)
= 1800


5.
a = -8
r = u2/u1 = 24/-8 = -3
Sn = ar pangkat n-1 ...n min 1 ya bukan kurang

S8 = ar^n-1
= -8 × -3^7
= -8 × -2.187
= 17.496

Bab Barisan dan Deret
Matematika SMP Kelas VIII

4] Aritmatika
b = (U11 - U5) / (11 - 5)
b = (42 - 18) / 6
b = 24/6
b = 4

a = U5 - 4b
a = 18 - 4 x 4
a = 18 - 16
a = 2

Sn = (n/2) x (2a + (n - 1) x b)

S30 = (30/2) x (2 x 2 + 29 x 4)
S30 = 15 x (4 + 116)
S30 = 15 x 120
S30 = 18.000

5] -8 + 24 - 72 + 216 ....
S8 = ?

r = U2 : U1
r = 24 : (-8)
r = -3

U5 = U4 x r
U5 = 216 x (-3)
U5 = -648

U6 = U5 x r
U6 = -648 x (-3)
U6 = 1.944

U7 = U6 x r
U7 = 1.944 x (-3)
U7 = -5.832

U8 = U7 x r
U8 = -5.832 x (-3)
U8 = 17.496

S6 = -8 + 24 - 72 + 216 - 648 + 1.944 - 5.832 + 17.496
S6 = 13.120