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Determina nr nat x stiind ca x+2.x+3.x+.....+10.x=385

Răspuns :

x+2x+3x+...+10x=385

1+2+3+...+10= 10*11:2= 5*11=55 (suma lui Gaus)
=> 55x=385
x=385:55
x=7