Question

[Phan Phương Linh]

GIẢI HỘ VS BẠN ƠI!!!!!!!!!!

a = 5 x 7 + 7 x 9 + 9 x 11 + 11 x 13 + 13 x 15 + 15 x 17 + 17 X 19 + ... + 99 x 101

Answer 1

[Nguyễn Ngọc Anh Minh]

6A=5.7.6+7.9.6+9.11.6+11.13.6+13.15.6+...+99.101.6

6A=5.7.(9-3)+7.9.(11-5)+9.11.(13-7)+11.13.(15-9)+13.15.(17-11)+...+99.101.(103-97)

6A=-3.5.7+5.7.9-5.7.9+7.9.11-7.9.11+9.11.13-9.11.13+11.13.15-11.13.15+13.15.17-...-97.99.101+99.101.103

6A=99.101.103-3.5.7 => A=(99.101.103-3.5.7)/6