Question

Tính A=1.2+2.3+3.4+ ... +n.(n+1)

Answer 1

3A=1.2.3+2.3.3+3.4.3+....+n.(n+1).3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+.....+n.(n+1).[n+2-(n-1)]

3A= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+n.(n+1).(n+2)-(n-1).n.(n+1)

3A=n.(n+1).(n+2)

A=n.(n+1).(n+2):3