6.
a) 1+2+3+...+58=
=58×59÷2= (suma Gauss)
=29×59=
=1711
b) (se face la fel)
1+2+...+37=
=37×38÷2=
=37×19=
=703
c) 2+4+6+...+90=
=2(1+2+3+...+45)=
=2×45×46÷2=
=45×46=
=2070
d) 1+5+10+...+70=
=1+5(1+2+3+...+12)=
=1+5×12×13÷2=
=1+5×6×13=
=1+390=
=391
7.
a) (1+2+3+...+90)+(3+6+9+...+45)=
=90×91÷2+3(1+2+3+...+15)=
=45×91+3×15×16÷2=
=4095+45×8=
=4095+360=
=4455
b) (42+43+44+...87)-(37+38+39+...+82)=
=42+43+...+87-37-38-...-42-43-...-82=
=83+84+85+86+87-37-38-39-40-41=
=425-(37+38+39+40+41)=
=425-195=
=230
Sper ca te-am ajutat!
