Question 1181280
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In this problem, there are two unknowns,  "a"  and  "b";  


so,  you need to construct two equations for these two unknowns.



There is the  STANDARD  WAY  to construct these two equations:


<pre>
    first  equation says that  {{{a[2]-a[1]}}} = {{{a[3]-a[2]}}}  (both sides represent the common difference)

    second equation says that  {{{a[3]-a[2]}}} = {{{a[4]-a[3]}}}  (both sides represent the common difference, again).
</pre>


It is the way on how @MathTherapy solves the problem in his post.



How @Mathlover1 and @josgarithmetic do it, &nbsp;it looks like chewing gum in the mouth 
instead of solving problem and instead of right teaching . . . 



You, &nbsp;probably, &nbsp;will solve tens of such problems in your life &nbsp;(in your school years).


THEREFORE, &nbsp;you need to know the right way and do not spend your time for nothing . . . 



To see many other similar &nbsp;(and different) &nbsp;problems solved, &nbsp;look into the lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/One-characteristic-property-of-arithmetic-progressions.lesson>One characteristic property of arithmetic progressions</A>

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Sequences-and-series/Finding-number-of-terms-of-an-arithmeti--progression.lesson>Finding number of terms of an arithmetic progression</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Sequences-and-series/Inserting-arithmetic-means-between-given-numbers.lesson>Inserting arithmetic means between given numbers</A> 

in this site.


Also, &nbsp;you have this free of charge online textbook in &nbsp;ALGEBRA-II &nbsp;in this site

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson>ALGEBRA-II - YOUR ONLINE TEXTBOOK</A>.


The referred lessons are the part of this online textbook under the topic 
<U>"Arithmetic progressions"</U>.