1.Tentukan jumlah 30 suku pertama dari deret berikut
A.8+11+14+17+...
B. 40+35+30+25+..
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A.8+11+14+17+...
B. 40+35+30+25+..
Note:
a = suku pertama
b = beda/ selisih
A.
8+11+14+17+....
a = 8
b = U2 - U1
= 11 - 8 = 3
b = 3
[tex] sn \: = \frac{n}{2} . \: (2a + (n - 1).b) \\ \\ [/tex]
[tex]s30 \: = \frac{30}{2} \times \: (2 \times 8 + (30 - 1 \times 3) \\ = 15 \times (16 + 29 \times 3) \\ = 15 \times (16 + 87) \\ = 15 \times (103) \\ s30 = 1.545[/tex]
B.
40+35+30+25+....
a = 40
b = 35-40 = -5
[tex]s30 = \frac{30}{2} \times (2 \times 40 + (30 - 1) \times - 5) \\ = 15 \times (80 + 29 \times - 5) \\ = 15 \times(80 - 145) \\ = 15 \times ( - 65) \\ = - 975[/tex]
[answer.2.content]
