Question

Jumlah enam suku pertama 120, jumlah tujuh suku pertama 133 suku ke 12nya?

Answer 1

U7 = S7-S6
a+6b = 133-120
a+6b = 13
a = 13 -6b
S6 = 6/2 (2a + 5b)
120 = 3 (2a + 5b)
2a + 5b = 40
2(13 - 6b) + 5b = 40
26 -12b + 5b = 40
-7b = 14
b = -2
a = 13 - 6(-2)
a = 13 + 12
a = 25
U12 = a + 11b
   = 25 + 11 (-2)
   = 3