tolong bantuannya no 58-60

Nomor 58 terlampir pada foto

59)
Sn = n/2 ( 2a + ( n- 1 ) b )
21 = 3/2 ( 2 a + ( 3 - 1 ) b )
42= 3 ( 2a + 2b )
14 = 2a + 2b ----------> [1]


Sn = n/2 ( 2a + ( n- 1 ) b )
78 = 6/2 ( 2 a + ( 6 - 1 ) b )
78 = 3 ( 2a + 25 )
26 = 2a + 5b -----------> [2]

2a + 2b = 14
2a + 5b = 26
eleminasi

- 3b = -12
b = 4

2a + 2b = 14
2a + 2(4) = 14
2a = 14-8
a = 3

Jumlah 10 suku pertama
Sn = n/2 ( 2a + ( n- 1 ) b )
Sn = 10/2 ( 2 a + ( 10 - 1 ) b )
Sn = 5 ( 2 × 3 + 9 × 4 )
Sn = 5 × 42
Sn = 210 ( B )