Question

1/1.3 + 2/3.7 + 3/7.13 + 4/13.21+ 5/21.35

Answer 1

Sửa đề:1/1*3+2/3*7+3/7*13+4/13*21+5/21*31

=1/2(2/1*3+4/3*7+6/7*13+8/13*21+10/21*31)

=1/2(1-1/3+1/3-1/7+...+1/21-1/31)

=1/2*30/31=15/31