Question

Dari barisan aritmatika diketahui u3=18 dan u7=38.jumlah 24 suku pertama adalah tolong cara menjawabnya

Answer 1

U3 = a + (3-1)b = 18 | U7 = a + (7-1)b = 38 -> U3 = a + 2b = 18 | U7 = a + 6b =38 | Lalu karena a-nya sama, maka U7 dikurangi U3 -> 4b = 20. Jadi b = 5. Lalu a -> U3 = a + (3-1) 5 = 18 -> a +(2) 5 = 18 -> a + 10 = 18 -> a = 8. S24 = n/2 (2a + (n-1) b) -> 24/2 (2 . 8 + 23 . 5) -> 12 (16 + 115) = 12 (131) = 1572.

Answer 2

U(n)= a + (n-1).b
U3= 18 => a + (3-1)b => a+2b = 18 |x3| => 3a+6b = 54
U7= 38 => a + (7-1)b => a+6b = 38 |x1| =>   a+6b = 38
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                                                               2a       = 16
                                                                 a       =  8
a+6b=38
8+6b=38
    6b=30
      b= 5

S(n) = n/2 . (2a+(n-1).b)
S24 = 24/2 . (2.8+(24-1).5)
S24 = 12.(16+23.5)
S24 = 12. (16+115)
S24 = 12.131
S24 = 1572