Calculati
a (1/21+1/14)-(1/35-1/42)+(1/14-1/21) b (4 întregi 7/9-1 întreg1/2) -(15/16-1/24) c (5/9+3/2+11/18)-(11/6-1/3-1/18)+(1/6+3/4+7/12) d. (7/9+17/10-1/18)-(13/27-1/18+7/9)+(1/10+1/15+1/9) va rog repede dau coroana.​