tentukan banyak suku dan jumlah deret aritmatika berikuta. 4+9+14+19+……+104b. -12-8-4-0-…….-128

A). b = 9 – 4 = 5
Un = a + (n-1)
104 = 4 + (n-1) 5
104 = 4 + 5n – 5
104 = 5n – 1
5n = 104 + 1
n= 105/5
n = 21

Sn = n/2 (a + Un)
S21 = 21/2 (4 + 104)
= 21/2 × 108
= 1134

b. -12-8-4-0-…….+128
b = -8 – (-12) = 4
Un = -12 + (n-1) 4
128 = -12 + (n-1) 4
128 = -12 + 4n – 4
128 = 4n – 16
-4n = -128 – 16
n= -144/-4
n = 36

Sn = n/2 (a + Un)
S36 = 36/2 (-12 + 128)
= 18 × 116
= 2016