# Arithmetic Sequences

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An arithmetic sequence is a series of numbers such that to get the next number in the sequence we add a number to the last term. We add the SAME number each time. For example

4, 9, 14, 19, 24 is an arithmetic sequence because we add 5 to each term to get the next term. The general form for the nth term in a geometric sequence is:

whereis the first term andis the difference between any two successive terms.

Thereflects the fact that to get the 1st term we don’t have to add anything: only from the 1st term do we start adding things.

When we add up n terms, we write down an expression like,

By writing this backwards we obtain

We can now add the two sequences, gettingon the left hand side and altogether n terms all the same,on the right hand side, so

We may be asked: The 3rd term of an arithmetic sequence is 9 and the 5th term is 17. Find the first term, the common difference and the smallest value of n such that

and

Now solve the simultaneous equations

(1)

(2)

Sub into (1)

Solve

Non integer or negative values of n are not allowed here, because we are considering only the natural numbers, so