Đáp án:
D =1.4+2.5+3.6+.......+55.58
D = 1. (2+2) +2.(2+3) +3.(2+4)+...+55.(50+8)
D = 1.2+1.2+2.2+2.3+2.3+3.4+...+2.99+55.58
D = (1.2+2.3+3.4+...+55.58) +2.(1+2+3+4+...+55)
*Gọi A= 1.2+2.3+3.4+...+55.58
3A = 3.(1.2+2.3+3.4+...+55.58)
3A = 1.2.3+2.3.3+...+99.100.3
3A = 1.2.3 +2 .3.(4-1)+...+55.58.(58-98)
3A = 1.2.3+2.3.4-1.2.3+...+ 55.158.101-55.57.100
3A = 55.57.101
3A = 3.16.100.101
A = 33.16.101
A = 332800
* Gọi B = 2. (1+2+3+4+...+55)
\__có 55 số hạng ___/
B= 2.[(1+55).55:2]
B = 2 .4950
B = 3080
A+B = 331600+3080 =337760
Vậy D =337760
