Bài 1:

a: Để A là phân số thì n-3<>0

=>n<>3

b: Thay n=0 vào A, ta được:

\(A=\frac{4}{0-3}=\frac{4}{-3}=-\frac43\)

Thay n=10 vào A, ta được:

\(A=\frac{4}{10-3}=\frac47\)

Thay n=-2 vào A, ta được:

\(A=\frac{4}{-2-3}=\frac{4}{-5}=-\frac45\)

Bài 2:

a: \(\frac{-4}{9}\cdot\frac{7}{15}+\frac{4}{-9}\cdot\frac{8}{15}\)

\(=\frac{-4}{9}\left(\frac{7}{15}+\frac{8}{15}\right)\)

\(=\frac{-4}{9}\cdot\frac{15}{15}=-\frac49\)

b: \(\frac{5}{-4}\cdot\frac{16}{25}+\frac{-5}{4}\cdot\frac{9}{25}\)

\(=\frac{-5}{4}\left(\frac{16}{25}+\frac{9}{25}\right)\)

\(=-\frac54\cdot\frac{25}{25}=-\frac54\)

c: \(4\frac{11}{23}-\frac{9}{14}+2\frac{12}{23}-\frac54\)

\(=\left(4+\frac{11}{23}+2+\frac{12}{23}\right)-\frac{9}{14}-\frac54\)

\(=7-\frac{9}{14}-\frac54=\frac{196}{28}-\frac{18}{28}-\frac{35}{28}=\frac{143}{28}\)

d: \(2\frac{13}{27}-\frac{7}{15}+3\frac{14}{27}-\frac{8}{15}\)

\(=\left(2+\frac{13}{27}+3+\frac{14}{27}\right)-\left(\frac{7}{15}+\frac{8}{15}\right)\)

=5+1-1

=5

e: \(11\frac14-\left(2\frac57+5\frac14\right)\)

\(=11+\frac14-2-\frac57-5-\frac14\)

\(=4-\frac57=\frac{23}{7}\)

g: \(\frac{7}{19}\cdot\frac{8}{11}+\frac{7}{19}\cdot\frac{3}{11}+\frac{12}{19}\)

\(=\frac{7}{19}\left(\frac{8}{11}+\frac{3}{11}\right)+\frac{12}{19}\)

\(=\frac{7}{19}+\frac{12}{19}=\frac{19}{19}=1\)

Bài 3:

a: \(\frac34\cdot\frac{16}{9}-\frac75:\frac{-21}{20}\)

\(=\frac{48}{36}+\frac75\cdot\frac{20}{21}\)

\(=\frac43+\frac{140}{105}=\frac43+\frac43=\frac83\)

b: \(2\frac13-\frac13\cdot\left\lbrack-\frac32+\left(\frac23+0,4\cdot5\right)\right\rbrack\)

\(=\frac73-\frac13\cdot\left\lbrack-\frac32+\frac23+2\right\rbrack\)

\(=\frac73-\frac13\cdot\left\lbrack\frac12+\frac23\right\rbrack=\frac73-\frac13\cdot\frac76=\frac73\left(1-\frac16\right)=\frac73\cdot\frac56=\frac{35}{18}\)

c: \(\left(20+9\frac14\right):2\frac14\)

\(=\left(20+9+\frac14\right):\frac94\)

=29,25:2,25

=13

d: \(\left(6-2\frac45\right)\cdot3\frac18-1\frac35:\frac14\)

\(=\left(6-2-\frac45\right)\cdot\frac{25}{8}-\frac85\cdot4\)

\(=\left(4-\frac45\right)\cdot\frac{25}{8}-\frac{32}{5}=\frac{16}{5}\cdot\frac{25}{8}-\frac{32}{5}=10-\frac{32}{5}=\frac{18}{5}\)

e: \(\frac{32}{15}:\left(-1\frac15+1\frac13\right)\)

\(=\frac{32}{15}:\left(-1-\frac15+1+\frac13\right)\)

\(=\frac{32}{15}:\left(\frac13-\frac15\right)=\frac{32}{15}:\frac{2}{15}=\frac{32}{2}=16\)

g: \(0,2\cdot\frac{15}{36}-\left(\frac25+\frac23\right):1\frac15\)

\(=\frac{3}{36}-\frac{6+10}{15}:\frac65\)

\(=\frac{1}{12}-\frac{16}{15}\cdot\frac56=\frac{1}{12}-\frac83\cdot\frac13=\frac{1}{12}-\frac89=\frac{3}{36}-\frac{32}{36}=-\frac{29}{36}\)

h: \(1\frac{13}{15}\cdot0,75-\left(\frac{8}{15}+0,25\right)\cdot\frac{24}{47}\)

\(=\frac{28}{15}\cdot\frac34-\left(\frac{8}{15}+\frac14\right)\cdot\frac{24}{47}\)

\(=\frac{21}{15}-\frac{32+15}{60}\cdot\frac{24}{47}=\frac{21}{15}-\frac{24}{60}=\frac75-\frac25=\frac55=1\)

g: \(5:\left(4\frac34-1\frac{25}{28}\right)-1\frac38:\left(\frac38+\frac{9}{20}\right)\)

\(=5:\left(\frac{19}{4}-\frac{53}{28}\right)-\frac{11}{8}:\frac{15+18}{40}\)

\(=5:\frac{80}{28}-\frac{11}{8}\cdot\frac{40}{33}=5\cdot\frac{28}{80}-\frac53=5\cdot\frac{7}{20}-\frac53=\frac74-\frac53=\frac{21-20}{12}=\frac{1}{12}\)

Bài 4:

a: \(x:3\frac{1}{15}=1\frac12\)

=>\(x:\frac{46}{15}=\frac32\)

=>\(x=\frac32\cdot\frac{46}{15}=\frac{23}{5}\)

b: \(5\frac47:x=13\)

=>\(\frac{39}{7}:x=13\)

=>\(x=\frac{39}{7}:13=\frac37\)

c: \(7x-3x=3,2\)

=>4x=3,2

=>x=0,8

h: \(\frac23x-\frac12x=\frac{5}{12}\)

=>\(x\left(\frac23-\frac12\right)=\frac{5}{12}\)

=>\(x\cdot\frac16=\frac{5}{12}\)

=>\(x=\frac{5}{12}:\frac16=\frac{5}{12}\cdot6=\frac52\)

i: \(2\frac14\left(x-7\frac13\right...