dari barisan artmetika diketahui suku ke3 = 14 dan suku ke7 = 26 ).jumlah 18 suku pertama adalah

diketahui :
U3 = 14
U7 = 26

ditanya :
jumlah 18 suku pertama ( S18 ) ?

jawab :
pertama, mencari ( a ) dan ( b )

Un = a + ( n - 1 ) × b

eliminasi
U3 = a + 2b = 14
U7 = a + 6b = 26
------------------------- (-)
- 4b = - 12
b = - 12 / - 4
b = 3

subtitusi
a + 2b = 14
a + 2 × 3 = 14
a + 6 = 14
a = 14 - 6
a = 8

[tex]sn = \frac{n}{2} \times (2a + (n - 1) \times b )\\ s18 = \frac{18}{2} \times (2 \times 8 + (18 - 1) \times 3) \\ s18 = 9 \times (16 + 17 \times 3) \\ s18 = 9 \times (16 + 51) \\ s18 = 9 \times 67 \\ s18 = 603[/tex]

u3 = a + (3 - 1) b = a + 2b
u7 = a + (7 - 1) b = a + 6b

14 = a + 2b
26 = a + 6b (dikurang)
-12 = -4b
b = 3
* 14 = a + 2b
14 = a + 2(3)
14 = a + 6
a = 14 - 6 = 8

u18 = a + (18 - 1) b = 8 + 17 x 3 = 59