diketahui baris aritmatika u5=8 dan u9=20 tentukan suku ke10 dan jumlah barisan ke10

Un = a + (n-1) b
U5 = a + (5-1)b = 8
U5 = a + 4b = 8
U9 = a + (9-1)b = 20
U9 = a + 8b = 20
U5 dikurangi U9 menjadi
-4b = -12
b = 3

a + 4b = 8
a + 4(3) = 8
a + 12 = 8
a = 8 - 12
a = -4

U10=?
U10 = a + (n-1) b
U10 = -4 + (10-1) 3
U10 = -4 + (9x3)
U10 = -4 + 27
U10 = 23

S10=?
Sn = n/2 (2a + (n-1) b)
S10 = 10/2 (2(-4) + (10-1) 3)
S10 = 5 ((-8) + (9x3))
S10 = 5 ((-8) + 27)
S10 = 5 (19)
S10 = 95