Question 1091988
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You have two numbers at the ends, 12 and -16.


When you insert 3 arithmetic means between them, you will have 4 intervals/segments between  -16  and  12.


The full length of the segment  [-16,12]  is  12 - (-16) = 12 + 16 = 28.


Hence, the length of each smallest interval/segment between the neighbor values will be  {{{28/4}}} = 7.


So, your sequense is 

-16, -16+7 = -9,  -9+7 = -2,  -2+7 = 5   and  5 +7 = 12.



<U>Answer</U>.  The three arithmetic means to insert are  -9,  -2,  and 5.
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Solved.