40+60+...120=
Dau inimioară


Răspuns :

40 + 60 + ...+120 =
= 20
·(2+3+..+6)
= 20
·[(6·7:2)-1]
= 20
·(21-1)
= 20·20
= 400
varianta 1
40+60+80+100+120=
40+120+60+100 +80=160+160+80=320+80=400

varianta 2
20(2+3+4+5+6)= 20(6*7/2-1)=20*(21-1)=20*20=400

varianta 3
40(1+0,5+2+2,5+3)=40*10=400