Question 1172920
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<pre>

    {{{(x-2)^2}}} + {{{x^2}}} + {{{(x+2)^2}}} = 980


    x^2 - 4x + 4 + x^2 + x^2 + 4x + 4 = 980


    3x^2 + 8 = 980


    3x^2 = 980 - 8 = 972


    x^2 = 972/3 = 324


    x = {{{sqrt(324)}}} = +/- 18.


<U>ANSWER</U>.   There are two such sequences. One is  16, 18, 20.

                                        The other is -20, -18  and -16.
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Solved.