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Sagot :
Answer:
The first term is -3.
Step-by-step explanation:
Arithmetic sequence:
In an arithmetic sequence, the difference between two consecutive terms is always the same, and it is called common difference d.
The nth term is given by:
[tex]a_n = a_1 + (n-1)d[/tex]
In which [tex]a_1[/tex] is the first term.
If we have the mth term instead of the first, we have:
[tex]a_n = a_m + (n-m)d[/tex]
We have the third term, and need the fifth and the sixth:
So we write both the fifth and the sixth in function of the third, then:
[tex]a_5 = a_3 + 2d = 5 + 2d[/tex]
[tex]a_6 = a_3 + 3d = 5 + 3d[/tex]
The sum of the fifth and sixth term of an AP is 30
We use this to find [tex]d[/tex]. So
[tex]a_5 + a_6 = 30[/tex]
[tex]5 + 2d + 5 + 3d = 30[/tex]
[tex]5d + 10 = 30[/tex]
[tex]5d = 20[/tex]
[tex]d = \frac{20}{5} = 4[/tex]
First term:
The third term is:
[tex]a_3 = a_1 + 2d[/tex]
Since [tex]a_3 = 5, d = 4[/tex]
[tex]a_1 = a_3 - 2d = 5 - 8 = -3[/tex]
The first term is -3.
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