bantuannya gan. (rumusanny yg tepat gan)

a(3)=a+2b=8
a(5)=a+4b=12
--------------------- -
-2b=-4
b=2
a+2b=8
a+2(2)=8
a+4=8
a=4

S(8)=[tex] \frac{8}{2} [/tex] (2a+(8-1)b)
=4 ((2x4) + (7x2))
=4x22
=88

Diketahui: 
u3=8
u5=12

Ditanya: s8?

Jawab:
u3 = a + 2b
u5 = a +4b

8 = a + 2b
12 = a + 4b -
-4 = -2b
b = 2
subtitusi ke persamaan u3
u3 = a + 2b
8 = a + 2.2
a = 8-4
a = 4

Sn = n/2 . [2a+(n-1)b]
S8 = 8/2 . [2.4 + 7.2]
S8 = 4. (8+14)
s8 = 4 . 22 = 88