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(2n+1)^{2}+(2n+3)^{2}+(2n+5)^{2}=(4n^{2}+4n+1)+(4n^{2}+12n+9)+(4n^{2}+20n+25)=12n^{2}+36n+35 a*1000+a*100+a*10+a*1=a*(1000+100+101+1)=a*1111 12n^{2}+36n+35=a*1111 \frac{12n^{2}}{a}\in C_{+}\\\frac{36n}{a}\in C_{+}\\ \frac{35}{a}\in C_{+}\Rightarrow a=\{1,5,7\} \\12n^{2}+36n+35=7*1111\\ 12n^{2}+36n+...