Question

1-2-3-...-30=?
Cum se rezolva?

Answer 1

Răspuns:

1-2-3-...-30=-(1+2+3+...+30)+2=-(30*31÷2)+2=-(15*31)+2=-465+2=-463

!!am pus +2 pt ca 1=-1+2

Answer 2

Răspuns:

1 - 2 - 3 - ... - 30 =

adunam si scadem 1

1 - 2 - 3 - .. - 30 - 1 + 1 =

= 2 - 1 - 2 - 3 - .. - 30

= 2 - (1 + 2 + 3 + .. + 30)

in paranteza avem o suma Gauss

aplicam formula 1 + 2 + .. + n = n(n + 1)/2

= 2 - (30×31/2)

= 2 - (15×31)

= 2 - 465

= - 463