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4. Calculaţi sumele: a) S=1+2+3+...+ 50
b) S = 2 + 4 + 6+ ... + 200
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Răspuns :

S = 1 + 2 + 3 + ... + 50 <=>

<=> S = 50 × 51 ÷ 2 <=>

<=> S = 2.550 ÷ 2 <=>

<=> S = 1.275

S = 2 + 4 + 6 + ... + 200 <=>

<=> S = 2 × ( 100 × 101 ÷ 2 ) <=>

<=> S = 2 × ( 10.100 ÷ 2 ) <=>

<=> S = 2 × 5.050 <=>

<=> S = 10.100

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