Question

tính 1.2+2.3+3.4+...+98.99

Answer 1

đặt A=1.2+2.3+3.4+...+98.99

=>3A=1.2.3+2.3.3+3.4.3+...+98.99.3

=1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)

=1.2.3+2.3.4-.1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99

=98.99.100=970200

=>A=323400

Vậy A=323400