\(\dfrac{7}{2\times3}+\dfrac{7}{3\times4}+\dfrac{7}{4\times5}+\dfrac{7}{5\times6}\)

= 7 x ( \(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}\) )

= 7 x \(\left(\dfrac{1}{2}-\dfrac{1}{6}\right)\)

= 7 x \(\dfrac{1}{3}=\dfrac{7}{3}\)

`4/7xx3/7+4/5xx6/7-4/5xx7/14`

`=4/7xx3/7+4/5xx(6/7-7/14)`

`=4/7xx3/7+4/5xx5/14`

`=12/49+2/7`

`=26/49`

=4/7x3/7+4/5x(6/7-7/14)

=4/7x3/7+4/5x5/14

=12/49+2/7

=26/49

Ta có: \(\frac{5-4}{2\times3}+\frac{6-5}{3\times4}+\frac{8-7}{4\times5}+\frac{9-8}{5\times6}\)

\(=\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}\)

\(=\frac12-\frac13+\frac13-\frac14+\frac14-\frac15+\frac15-\frac16=\frac12-\frac16=\frac13\)

a, 24/35

b, 13/7

k cho mih

Chọn đáp án D

0,6 

3/5x6/7+3/5x16/14-3/5x1

=3/5x(6/7+16/14-1)

=3/5x1

=3/5

  7-4-2+5x3+5x6

=3-2+5x(3+6)

=1+5x9

=1+45

=46

7 - 4 - 2 + 5 x 3 + 5 x 6

=3 - 2 + 5 x ( 3 + 6 )

=1 + 5 x 9

=1 + 45

=46

1/5 x 6 +7/6-6/5

=1/30+7/6-6/5

=1/30+35/30-36/30

=0/30

=0

    1/5x6 + 7/6 - 6/5 = 1/30 + 7/6 - 6/5

                               = 1/30 + 35/30 - 36/30

                               = 36/30 - 36/30

                               =0/30 = 0