Question 1203795
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Between the 2nd and the 6th terms of an AP, there are  6-2 = 4 gaps of equal length on the number line.


Each gap is  {{{(40-12)/4}}} = {{{28/4}}} = 7 units long.


The size of each gap is exactly the common difference value.


So, the common difference of this AP is  7  units.
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Simple, short and elegant reasoning.


It makes sense to memorize this mantra an use it
in many other similar problems to provide quick solutions.