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cat face 5+10+15+20+...+500=? a)50 500 b)25 250 c)49 995 d)50 250

Răspuns :

S=5+10+15+20+....+500
S=5(1+2+3+...+100)
S=5 × 
S=5 ×
S=5 ×5050
S=25250
[(5+500)×100]:2=(505×100):2=50.500:2=25.250
5=5×1
10=5×2
15=5×3
20=5×4
........
500=5×100
regulă: 1 2 3 4....100
100-1+1=100

RĂSPUNS : B