Question

Tentukan jumlah setiap deret berikut
a. 6+11+16+21+....+101
b. 7+11+15+19+23+....+103

Answer 1

Yang a dulu
Un = a+ (n-1)b
101 = 6 + 5n - 5
100 = 5n
n = 20
b.
Un = 7 + 4n -4
103 = 4n + 3
100 = 4n
n = 25

Answer 2

A. a= 6; b = 11-6 = 5
Un = a+ (n-1) b
101 = 6 + (n-1) 5
101 = 6 + 5n -5
101 = 1 + 5n
101-1 = 5n
100 = 5n
20 = n

S20 = 20/2 {2.6 + (19)5}
       = 10 {12 + 95}
       = 10 x 107 = 1070

b. a = 7; b = 11-7 = 4
rumus sama seperti diatas
103 = 7 + 4n -4
103 = 3 +4n
100 = 4n
25 = n

S25 = 25/2 {2.7 + (24) 4}
         25/2 {14 + 96}
         25/2 x 110 = 2750/2 = 1375

semoga benar :)