1+2+3...+2011+2012+2011...+3+2+1
=(1+2+3...+2011+2012)+(2011...+3+2+1)
=(1+2012)x2012÷2+(1+2011)x2011÷2
=2013x2012÷2+2012x2011÷2
=(2013+2011)x2012÷2
=2012x2012
=2012²