a) 1/3+1/3: (1-1/3) egal 5/6
1) 1-1/3 egal 2/3
2) 1/3 x3/2 egal 1/2
3) 1/3+ 1/2 egal 5/6
b) 28/31 x [1/4 + (2 x 1/2 - 7/4) :7/9] egal -20/31
1) 2 x 1/2 egal 1
2) 1-7/4 egal - 3/4
3) - 3/4 x 9/7 egal - 27/28
4) 1/4+(-27/28) egal -20/28
5) 28/31x(-20/28) egal -20/31
