4. Efectuați: a)-2-7-9+1+6-8+4+3+4+2+3-945;
b)+ 6-8+2-4+9+2+4+2-5-8-1+7+3; c)+8-4-947-1-2-4-5+7-3+2+6+ 1; d) +8+5-4-7-6-9-2+3+9-1+2+3+4; e)-6+2-9+7+2+3+4+8+3-9-4+1-5;
f)+8+2-3-6+1+3+4+2-8-5+9-7-2.​