Question

Dari barisan aritmetika diketahui suku ke -7 =22 dan suku ke-11 =34 . jumlh 18 suku pertama adalh
a . 531
b . 666
c , 1.062
d . 1.332

Answer 1

22 = a +   6b
34 = a + 10b  -
-12 = -4b
-12 : -4 = b
3 = b

maka, a
22 = a + 6b
22 = a + 6(3)
22 = a + 18
22 - 18 = a
4 = a

maka, S18
18/2 x [(2 x 4 ) + (18-1)3]
18/2 x [8 + (17 x 3)]
9 x [8 + 51]
9 x 59 = 531

Answer 2

B (beda) : U11 - U7 = 34-22 = 12  = 3
                   11-7           4        4
a : U7 = a+(n-1)b
     22 = a+6(3)
     22 = a+18
       a = 22-18 = 4

S18 = 1/2n(2a+(n-1)b)
S18 = 1/2×18 (2×4 + 17×3)
               = 9 (8+51)
                = 9(59) = 531