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S=2+4+6+8+.....+238+239+240

Răspuns :

S=2+4+6+8+.....+238+240
S=2* (1+2+3+4+...+119+120)
S=2*[tex]\frac{120*121}{2} [/tex]   (doi cu doi se simplifica)
S=120*121
S=14520
[tex]\displaystyle 2+4+6+8+...+238+240=2(1+2+3+...+119+120)= \\ \\ =2 \times \frac{120(120+1)}{2} =2 \times \frac{120 \times 121}{2} =\not 2 \times \frac{14520}{\not 2} =14520[/tex]