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a) 20-19+18-17+16-15+......+2-1 b) 111-110+109-108+...+3-2+1

Răspuns :

a= (20-19) + (18-17) +(16-15) +......+(2-1) = 1+1+1+1+....+1 = 1·10 = 10
b = (111-110) + (109-108) +......+(3-2) + 1 = 1·55 +1 = 56