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2014-11-10T09:09:07+02:00
12 x+1+2+...+11=78; 12x +12•11:2=78; 12x+66=78; 12x=12 x=1
2 5 2
2014-11-10T09:09:21+02:00
X+x+1+x+2+...+x+11=78
12x+(11x12)/2=78
12x+66=78| -66
12x=12 | :12
x=1
2 5 2