Question

[Trang Nguyễn]

Bài 1:tính

a, A = 25/6+25/6.11+25/11.16+...+25/41.46

b, B= 1/10+1/15+1/21+...+1/120

Bài 2: Chứng minh rằng:

1/2+1/3.4+1/5.6+...+1/49.50=1/26+1/27+1/28+...+1/50

bài 3: tìm y

(y - 1/2).(1/2+1/6+1/12+1/20+.....+1/90) = 1/3

Answer 1

[Vương Thị Diễm Quỳnh]

b1/A=25/1.6+25/6.11+25/11.16+....+25/41.46

=5.(5/1.6+5/6.11+5/11.16+...+5/41.46)

=5.(1/1-1/6+1/6-1/11+1/11-1/16+....+1/41-1/46)

=5.(1/1-1/46)

=5.45/46

=225/46