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1×2+2×3+3×4+4×5+5×6+6×7 cum rezolv dând factor comun?

Răspuns :

1x2+2x3+3x4+4x5+5x6+6x7= 
2+3+12+20+30+42=
5+32+72=
37+72
109
1x2+2x3+3x4+4x5+5x6+6x7= 
2+3+12+20+30+42=
5+32+72=37+72=109