B bằng 1.2+3.4+5.6+…+99.100

Ta có:

`B = (2 – 1).2 + (4 – 1).4 + (6 – 1).6 + … + (100 – 1).100`

`B= 22 – 2 + 42 – 4 + 62 – 6 + … + 1002 – 100`

`B= (22 + 42 + 62 + … + 1002) – (2 + 4 + 6 + … + 100)`

`B= 22.(12 + 22 + 32 + … + 502) – (100 + 2).50 : 2`

`B= 22.50.51.52 : 6 – 51.50`

  ` = 88400 – 2550`

   ` = 85850`