ngay đó mk mới làm xong
đừng để ý mấy dấu chấm
ta chia A cho 2
=> A/2=1x2+2x3+3x4+.......+99x100
a=66660
Đặt A = 4 + 12 + 24 + 40 +......+ 19404 + 19800
\(\frac{1}{2}A=2+6+12+...+9702+9900\)
\(\frac{1}{2}A=1.2+2.3+3.4+...+98.99+99.100\)
\(\frac{3}{2}A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+98.99.\left(100-97\right)+99.100.\left(101-98\right)\)
\(\frac{3}{2}A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100\)
A = ( 99.100.101 )\(.\frac{2}{3}\)
A = 666600
Ta chia A cho 2
=> A/2 = 1 x 2 + 2 x 3 + 3 x 4 + .......... + 99 x 100
A = 66660