B
bintut97


a, 1.1.1+2.2.2+3.3.3+......+n.n.n
b, 1.2+2.3+3.4+.....+n.(n+1)
c, 1.2.3+2.3.4+........+n.(n+1).(n+2)
Hay rut gon cac bieu thuc tren.
giup minh voi
b, 1.2+2.3+3.4+.....+n.(n+1)
c, 1.2.3+2.3.4+........+n.(n+1).(n+2)
Hay rut gon cac bieu thuc tren.
giup minh voi