Question

tinh nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/91x94

 

Answer 1

1/1.4+1/4.7+1/7.10+...+1/91.94

=1/3.(3/1.4+3/4.7+3/7.10+...+3/91.94)

=1/3.(1-1/4+1/4-1/7+1/7-1/10+...+1/91-1/94)

=1/3.(1-1-94)

=1/3.(93/94)

=31/94

Answer 2

Đặt A= 1/1*4+1/4*7+1/7*10+....+1/91*94

3A= 3/1*4+3/4*7+3/7*10+....+3/91*94

3A=1/1-1/4+1/4-1/7+1/7-1/10+............+1/91-1/94

3A=1-1/94=93/94=>A=93/94*1/3=31/94

=31/94 k mình nha bạn 

Answer 3

1/1x4+1/4x7+1/7x10+..+1/91x97

=1-1/4+1/4-1/7+1/7-1/10+...-1/91+1/97

=1-1/97

=96/97