Dari barisan aritmarika suk ke 2 =8 dan suku ke 4 =18. Tentukan jumlah suku ke 26

Un = bn + a - b
U2 = 2b + a - b
8    = b + a

Un = bn + a - b
U4  = 4b + a - b
18  = 3b + a 

8 = b + a
18 = 3b + a
---------------- -
-10 = -2b
    b = -10/-2
    b = 5

8 = b + a
8 = 5 + a
a = 8 - 5
a = 3

  Sn = n/2 (2a + (n-1)b)
S26 = 26/2 (2.3 + (26-1)5)
        = 13 ( 6 + 125)
        = 13 x 131
        = 1.703

[tex]U_{2} = 8 =\ \textgreater \ a+b = 8 \\ U_{4} = 18 =\ \textgreater \ a+3b = 18\\ -2b = -10\\ b = 5\\ a+5 = 8\\ a=3 \\\\ maka \\\\ S_{26} = \frac{n}{2} (2a+(n-1)b)\\\\ = \frac{26}{2} (3 + (26-1)5 \\\\ = 26 (2.3 + (25.5))\\\\ = 13.131 \\ = 1703[/tex]
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