7. Calculaţi: a) 9-8+7 - 6+ ... +3-2 +1 -0; b) 10 - 9+8-7+ ... +2-1+0; c) 10 - 9+9 - 8+8-7+ ... +3-2 +2 -1; d) 16+15-14-13 + 12 +11 - 10 - 9+ ... +4+3 -2 -1​