Metoda guess
2+4+6+.........+140=?






Răspuns :

[tex]2 + 4 + 6 + ... + 140[/tex]

[tex] = 2(1 + 2 + 3 + ... + 70)[/tex]

[tex] = 2 \times \frac{70(70 + 1)}{2} [/tex]

[tex] = 70(70 + 1)[/tex]

[tex] = 70 \times 71[/tex]

[tex] = 4970[/tex]