A

Gọi số đó là ab (a,b là chữ số; a khác 0)

Theo bài ra ta có:

     ab-ba=n² (Với nϵN)

⇒ a.10+b-b.10-a = n²

⇒ 9a-9b = n²

⇒ 9.(a-b)=n²

⇒ a-b=9 ⇒ a=9,b=0 (vì a,b đều bé hơn 10)

Vậy số cần tìm là 90

\(\dfrac{1}{3}+\dfrac{2}{3}+\dfrac{4}{3}+\dfrac{19}{3}=\dfrac{1+2+4+19}{3}=\dfrac{26}{3}\)

\(\dfrac{3}{4}+\dfrac{4}{4}+\dfrac{5}{4}+\dfrac{6}{4}+\dfrac{x}{4}+\dfrac{8}{2}+\dfrac{9}{4}\)

=\(\dfrac{3}{4}+\dfrac{4}{4}+\dfrac{5}{4}+\dfrac{6}{4}+\dfrac{x}{4}+\dfrac{16}{4}+\dfrac{9}{4}\)

=\(\dfrac{3+4+5+6+x+16+9}{4}=\dfrac{43+x}{4}\)

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

⇒\(\dfrac{2}{3}:x=\dfrac{-1}{5}-\dfrac{-2}{7}=\dfrac{3}{35}\)

⇒\(x=\dfrac{2}{3}:\dfrac{3}{35}=\dfrac{70}{9}\)

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

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

⇒\(x=\dfrac{1}{6}:\dfrac{7}{4}=\dfrac{1}{6}.\dfrac{4}{7}=\dfrac{2}{21}\)

100 - 2(x-4) = 20

⇒ 2(x-4)= 100-20=80

⇒ x-4 = 80:2 = 40

⇒ x= 40+4=44