Question

Tính A= 13.15+15.17+17.19+...+99.101

Answer 1

6A=13.15.6+15.17.6+...+99.101.6

6A=13.15(17-11)+15.17(19-13)+...+99.101(103-97)

6A=(13.15.17-11.13.15)+(15.17.19-13.15.17)+...+(99.101.103-97.99.101)

6A=99.101.103-11.13.15

6A=1027752

A=171292

Answer 2

Ta co:13.15+15.17+....+99.101

Suy ra 1/A=1/13.15+15.17+.....+99.101

=1/13.15+1/15.17+....+1/99.101

Suy ra 1/A.2=1/2.A=2.(1/13.15+1/15.17+....+1/99.101)

=2/13.15+2/15.17+....+2/99.101

=1/13-1/15+1/15-1/17+....+1/99-1/101

=1/13-1/101

=101-13/1313

=87/1313