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x+11+22+33+...+77=2x-187

Răspuns :

x+11(1+2+...+7)=2x-187
x+11*7*8/2=2x - 187 - e formula sumei Gauss
11*7*8/2+187=2x -x 
308+187=x
x=495
Simple metoda sumei gaus:
x+11(1+2+...+7)=2x-187
x+11*7*8/2=2x - 187
11*7*8/2+187=2x -x 
308+187=x
x=495