Bài 1: 

a: \(\dfrac{25}{42}-\dfrac{20}{63}=\dfrac{75-40}{126}=\dfrac{35}{126}=\dfrac{5}{18}\)

b: \(\dfrac{9}{20}-\dfrac{13}{75}-\dfrac{1}{6}=\dfrac{135}{300}-\dfrac{52}{300}-\dfrac{50}{300}=\dfrac{33}{300}=\dfrac{11}{100}\)

6 2/7  + 7 3/5 + 8 6/9 + 9 1/4 + 2/5 +  5/7 + 1/3  x 3/4 + 1967

= 44/7 + 38/5 + 78/9 + 37/4 + 2/5 + 5/7 + 1/3 + 1967

= ( 44/7 + 5/7 ) + ( 38/5 + 2/5 ) + ( 26/3 + 1/3 ) + ( 37/4 + 3/4 )  +1967

=  7 + 8 + 9 + 10  + 1967

= 15  + 9 + 10 + 1967

= 24 + 10 + 1967

= 34 + 1967 

= 2001

tính nhanh lớp 6 20 nhân 17-4 nhân 5 nhân 7

\(a.2\left(x-1\right)^2+\left(x+3\right)^2=3\left(x-2\right)\left(x+1\right)\)

\(\Leftrightarrow2x^2-4x+2+x^2+6x+9=3x^2-3x-6\)

\(\Leftrightarrow2x^2+x^2-3x^2-4x+6x+3x+2+9+6=0\)

\(\Leftrightarrow5x+17=0\)

\(\Leftrightarrow x=-\dfrac{17}{5}\)

KL.............

\(b.\left(x+2\right)^2-2\left(x-3\right)=\left(x+1\right)^2\)

\(\Leftrightarrow x^2+4x+4-2x+6=x^2+2x+1\)

\(\Leftrightarrow x^2-x^2+4x-2x-2x+4+6-1=0\)

\(\Leftrightarrow9=0\left(vôly\right)\)

KL..................

\(c.TươngTự\)

ta có : x+3/5+x+4/5=x+5/3+x+6/2

   => (x+x)+(3/5+4/4)=(x+x)+(5/3+6/2)

   => 2x+8/5=2x+14/3  ( vô lí ) 

Ta có: \(x+\frac{3}{5}+x+\frac{4}{4}=x+\frac{5}{3}+x+\frac{6}{2}\)

=> \(x+\frac{3}{5}+x+1=x+\frac{5}{3}+x+3\)

=> \(x+1+x+3=x+\frac{5}{3}-x-\frac{3}{5}\)

=> \(2x+4=\frac{25-9}{15}\)

=> \(2x+4=\frac{16}{15}\)

=> \(2x+4=1+\frac{1}{15}\)

=> \(2x+4-1=\frac{1}{15}\)

=> \(2x+3=\frac{1}{15}\)

=> \(2x=\frac{1}{15}-3\)

=> \(2x=\frac{1-45}{15}\)

=> \(2x=-\frac{44}{15}\)

=> \(x=\frac{\left(-\frac{44}{15}\right)}{2}\)

=> \(x=-\frac{44}{15}.\frac{1}{2}\)

=> \(x=-\frac{22}{15}\)

[x+1]+[x+2]+[x+3]+[x+4]+[x+5]=45

=[x+x+x+x+x]+[1+2+3+4+5]=45

=5x+15=45

5x=30

x=30:5

x=6

Vậy x=6

(x+1)+(x+2)+(x+3)+(x+4)+(x+5)=45

(x+x+x+x+x)+1+2+3+4+5=45

5x+15=45

5x=45-15

5x=30

x=30:5

x=6

vậy x=6