Question 1109037
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<pre>
   (x-a)^2 + (x+b)^2 + (x-c)^2 +x^2  =  (open parentheses) = 

=  (collect like terms linear in  "x" ) = 

= (next use the given identity) = a^2 + b^2 + c^2.
</pre>

QED.