Question

calculeaza 1+2+3-4+...99-100

Answer 1

1+2+3-4+5+6+7-8+...+97+98+99-100=
1+2+3+4-4-4+5+6+7+8-8-8...+97+98+99+100-100-100=
1+2+3+4+5+6+7+8+...+97+98+99+100-2x(4+8+12+...+100)=
(100x101):2-2x4(1+2+3+...+25)=
50x101-2x4x25x26:2=
5050-100x26=5050-2600=2450
am folosit formula de adunare a primelor n nr, naturale, adica:
1+2+3+...+(n-1)+n=nx(n+1):2

Answer 2

1+3=3
3+3=6
6-4=2
100-99=1
2+1=3