Đáp án:
Giải thích các bước giải:
$\text{A=1x2x3+3x4x5+.........+48x49x50}$
$\text{=> A.4=1.2.3.4+3.4.5.4+...+48.49.50.4}$
$\text{=>A.4=1.2.3.(4-0)+3.4.5.(6-2)+...+48.49.50.(51-47)}$
$\text{A.4=1.2.3.4-0.1.2.3+3.4.5.6-2.3.4.5+...+48.49.50.51-47.48.49.50}$
$\text{A.4=48.49.50.51}$
$\text{=> A= 12.49.50.51}$