Tentukan l jika 2 + 5 + 8 + ... + l = 1000

Mapel : Matematika
Kelas : IX SMP
Materi : Barisan dan deret

Pembahasan :
2 + 5 + 8 + ... + l = 1000

Un = l
a + (n - 1)b = l
2 + (n - 1)3 = l
2 + 3n - 3 = l
3n - 1 = l

Sn = 100
n/2(a + Un) = 100
n(2 + 3n - 1) = 200
n(3n + 1) = 200
3n² + n = 200
3n² + n - 200 = 0
(3n + 25)(n - 8) = 0
n = 8

Un = l
U8 = l
a + 7b = l
2 + 7(3) = l
2 + 21 = l
l = 23