Question

Tính nhanh:

A = 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56

Answer 1

 A = 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8         

    = 1/2 - 1/3 + 1/3 -1/4 + 1/4 - 1/5 + 1/5 - 1/6 +1/6 - 1/7 + 1/7 - 1/8 

    = 1/2 - 1/8           

     =  3/8        

Answer 2

A=1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

=1/2-1/3+1/4-1/5+1/5-1/6+1/7-1/8

=1/2-1/8

=4/8-1/8

=3/8

Answer 3

A=1/2.3+1/3.4+...+1/7.8

A=1/2-1/3+1/3-1/4+...+1/7-1/8

A=1/2-1/8

A=3/8

Answer 4

A=1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

=1/2-1/3+1/4-1/5+1/5-1/6+1/7-1/8

=1/2-1/8

=4/8-1/8

=3/8