Giải:

\(B=\dfrac{3}{3\times5}+\dfrac{3}{5\times7}+\dfrac{3}{7\times9}+...+\dfrac{3}{48\times50}\) 

\(B=\dfrac{3}{2}\times\left(\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+...+\dfrac{2}{48\times50}\right)\) 

\(B=\dfrac{3}{2}\times\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{48}-\dfrac{1}{50}\right)\) 

\(B=\dfrac{3}{2}\times\left(\dfrac{1}{3}-\dfrac{1}{50}\right)\)

\(B=\dfrac{3}{2}\times\dfrac{47}{150}\) 

\(B=\dfrac{47}{100}\) 

Chúc em học tốt!

Đặt \(A=\frac{5}{4.6}+\frac{5}{6.8}+.....+\frac{5}{48.50}\)

\(\Leftrightarrow\frac{5}{2}A=\frac{2}{4.6}+\frac{2}{6.8}+\frac{2}{8.10}+.....+\frac{2}{48.50}\)

\(\Leftrightarrow\frac{5}{2}A=\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+....+\frac{1}{48}-\frac{1}{50}\)

\(\Leftrightarrow\frac{5}{2}A=\frac{1}{4}-\frac{1}{50}\)

\(\Leftrightarrow\frac{5}{2}A=\frac{23}{100}\Rightarrow A=\frac{23}{100}.\frac{2}{5}=\frac{23}{250}\)

Ta có : -55 . 75 + 13.(-75) + 75 . (-32)

= (-55 - 13 - 32). 75

= -100 . 75

= -7500

75.[(-55)+(-13)+(-32]

75.(-100)

-7500

=-55 . 75 + (-13 ). 75 + 75. (-32 )

=75. ( -55+(-13) +(- 32))

=75.-100

=-7500

Hok tốt

\(\frac{75}{100}+\frac{18}{21}+\frac{19}{32}+\frac{1}{4}+\frac{3}{21}+\frac{13}{32}\)

\(=\left(\frac{75}{100}+\frac{1}{4}\right)+\left(\frac{18}{21}+\frac{3}{21}\right)+\left(\frac{19}{32}+\frac{13}{31}\right)\)

\(=1+1+1\)

\(=3\)

Có những phân số chưa tối giản. Tuy nhiên vì đây là bài tính nhanh nên sẽ không rút gọn hết. Xíu quy đồng lại nữa :( Nên chọn lựa ra thôi.

\(\frac{75}{100}=\frac{3}{4}\)

Viết lại có: \(\frac{3}{4}+\frac{18}{21}+\frac{19}{32}+\frac{1}{4}+\frac{3}{21}+\frac{13}{32}\)

          \(=\left(\frac{3}{4}+\frac{1}{4}\right)+\left(\frac{18}{21}+\frac{3}{21}\right)+\left(\frac{19}{32}+\frac{13}{32}\right)\)

          \(=1+1+1\) 

          \(=3\)

= 3 nha banj

75/100+18/21+19/32+1/4+3/21+13/32

=(75/100+1/4)+(18/21+3/21)+(19/32+13/32)

=1+1+1=3

75/100+18/21+19/32+1/4+3/21+13/32

=1/4+75/100+19/32+13/32+3/21+18/21

=1+1+1=3

minh cho ban 1 k nhe 

75/100+18/21+19/32+1/4+3/21+13/32=3 ban nhe!

ung ho nhe!

=3 nha bạn

nếu đung k mình nha

kb vs mk rùi mk giải cho

32 . 75 + 68 . 84 + 65 . 32 + 68 . 56

= 32 . ( 75 + 65 ) + 68 . ( 84 + 56 )

= 32 . 140 + 68 . 140

= 140 . ( 32 + 68 )

= 140 . 100

= 14000

32 x 75 + 68 x 84 + 65 x 32 + 68 x 56

= ( 32 x 75 ) + ( 65 x 32 ) + ( 68 x 84 ) + ( 68 x 56 )

= 32 x ( 75 + 65 ) + 68 x ( 84 +56 )

= 32 x 140 + 68 x 140

= 140 x ( 32 + 68 ) 

= 140 x 100

= 14000

[​32.(75+65)]+[68.(56+84)]

=32.140+68.140

=140.(68+32)

=140.100

=14000