1/3+1/7+ x/21=2/21
7×1+3×1 totul supra /21+ x/21=2/21
10/21+x/21=2/21
x/21=2/21 -10/21
x/21=-8/21
x= -8/21 ×21/1
x= -168/21 simplificat cu 21
x= -8
proba
1/3+1/7+ (-8)/21=
=7×1 + 3×1 + (-8) totul supra/21=
=7+3+(-8) totul supra/21=
=10+ (-8) totul supra /21 =
=2/21 ( Adevarat )
