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Calculați sumele:
a) 1+2+3+...+99
b) 2+4+6+...+198
Urgent!!


Răspuns :

[tex]\displaystyle a).1+2+3+...+99= \frac{99(99+1)}{2} = \frac{99 \times 100}{2} = \frac{9900}{2} = 4950 \\ \\ b).2+4+6+...+198=2(1+2+3+...+99)=2 \times \frac{99(99+1)}{2} = \\ \\ =2 \times \frac{99 \times 100}{2} =\not2 \times \frac{9900}{\not2} =9900[/tex]