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altavistard
One way to do this problem is to determine the common difference.  If the 5th and 8th terms are -9 and -21, we can do this by subtracting -9 from -21:

-21-(-9) = -12.  The 5th and 8th terms are not consecutive, so we have to think in terms of (8-5), or 3, times the common difference to get from -9 to -21.

Note that -12 divided by 3 is -4.  Thus, the common difference is -4.

Check:  -9 - 4 = -13; -13 - 4 = -17; -17 - 4 = -21 (which is correct).

We know that the 5th term is -9.  To find the 4th term, work backwards:  subtract (-4) from -9, which produces -9+4=-5.

The fourth term is -5.  Subtracting -4 from this (which is the same as adding 4 to this -5) produces the third term; it is -1.  Can you now find the 2nd and 1st terms using the same approach?

ashishdwivedilVT

The first four terms of the Arithmetic Sequence is 7, 3, -1 and -5.

What is Arithmetic Progression?

An arithmetic progression is a sequence where the differences between every two consecutive terms are the same.

Here, We know that, nth term of AP

        Aₙ = A + (n-1) D

          5th term A₅ = -9 = A + 4D  ...........(i)

         and 8th term A₈ = -21 = A + 7D    ............(ii)

From eq. (i) and (ii), we get

A = 7     D = -4

Thus, the first four terms of the sequence are 7, 3, -1 and -5.

Learn more about Arithmetic Progression from:

https://brainly.com/question/24873057

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