D5050 (100-99)*(100+99)+(98-97)*(98+97)+.+(2-1)*(2+1)
=1*(100+99)+1*(98+97)+.1*(2+1)
=100+99+98+97+.+2+1
=(100+1)+(99+2)+.+(51+50)
=101 * 50
=5050
D5050 (100-99)*(100+99)+(98-97)*(98+97)+.+(2-1)*(2+1)
=1*(100+99)+1*(98+97)+.1*(2+1)
=100+99+98+97+.+2+1
=(100+1)+(99+2)+.+(51+50)
=101 * 50
=5050