Question 1135842
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Here is a quick informal solution, if an algebraic solution is not required.<br>
The middle term in an AP with three terms is the average of the terms.  If the sum of the three terms is 21, then the middle term is 7.<br>
The given information then says the product of the first and third terms is 7+6=13.  Since 13 = 1*13, the first and last terms are (in either order) 1 and 13.<br>
ANSWERS:  1, 7, 13  or  13, 7, 1