Question

 Tính: 4/5.7 + 4/7.9 + ... + 4/59.61 

Answer 1

4/5.7+4/7.9+...+4/59.61

=2.(2/5.7+2/7.9+...+2/59.61)

=2.(1/5-1/7+1/7-1/9+...+1/59-1/61)

=2.(1/5-1/61)

=2.56/305

=112/203

Answer 2

A=( 2/5.7+2/7.9+.........+2/59.61).2

 A = (1/5-1/7+1/7-1/9+.......+1/59-1/61).2

A= (  1/5-1/61)2

Answer 3

B=4/5.7+4/7.9+4/9.11+...+4/59.61