Question

Sa se calculeze 1+5+9+......+2001

Answer 1

1+5+9+...+2001 =

= 1+(4•1+1)+(4•2+1)+...+(4•500+1)

= 1+4•(1+2+3+...+500) + 1•500

= 501 + 4•(500•501)/2

= 501•2•500•501

= 501•(1+2•500)

= 501•1001

= 501501