u2=8 u4=14 s6=? aritmatika

diket : u2=8
u4=14
dit : s6=?
jawab :
¤ b=(u4-u2) / (4-2)
b=(14-8) / 2
b=6 / 2
b=3
¤ u2 = a+b = 8
a+3 = 8
a = 8 - 3
a = 5
¤ Sn = n/2 x (2a+(n-1)b)
S6= 6/2 x (2(5) + (6-1) 3)
= 3 x (10 + (5x3))
= 3 x (10+15)
= 3 x 15
= 45

u2 = a + b = 8
u4 = a + 3b = 14
_______________ -
             -2b = -6
                b = 3
a + b = 8
a + 3 = 8
a = 5

Sn = n/2 (2a + (n-1) b)
S6 = 6/2 (2(5) + (6-1)(3))
S6 = 3 (10 + 5(3))
S6 = 3 (10 + 15)
S6 = 3 (25)
S6 = 75