Question

grupeaza convenabil termenii 1+2+3+4+5+6+...+14+15+16+17+18+19+20=_

Answer 1

1+2+3+....+18+19+20=
=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)=
=20*9+30=
=180+30=
=210

Answer 2

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20=
(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+10+20
20+20+20+20+20+20+20+20+20+10+20=
40+40+40+40+50=
80+80+50=
160+50=210