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Va roog! Rapid !!!!...

Va Roog Rapid class=

Răspuns :

x=1/4×1/15+1/4×2/15+1/4×3/15+1/4×4/15+....+1/4×15/15=
=1/4×(1/15+2/15+3/15+4/15+....+15/15)=
=1/4×(1+2+3+4+....+15)/15=
=1/4×(15×16)/(15×2)=
=1/4×8=2