Ta có: n.(n+3)=n.(n+1)+2nn.(n+3)=n.(n+1)+2n
⇒ A=1.4+2.5+...+48.51A=1.4+2.5+...+48.51
= 1.2+2.3+...+48.49+2.1+2.2+...+2.481.2+2.3+...+48.49+2.1+2.2+...+2.48
= 48.49.503+2.(1+48).[(48−1):1+1]:248.49.503+2.(1+48).[(48−1):1+1]:2
= 39200+235239200+2352
= 41552 đây nha hải