Question

[Dương Linh Đan]

so sánh các cặp phân số sau (giải thích cách làm);
a 11/14;13/16
b 2019/2020;2020;2021
c 205;207/2019;2021
d 99/100;97/98

Answer 1

[Nguyễn Đăng Nhân]

a)
Ta có:
\(\dfrac{11}{14}=1-\dfrac{3}{14}\)
\(\dfrac{13}{16}=1-\dfrac{3}{16}\)
So sánh phân số \(\dfrac{3}{14}\) và \(\dfrac{3}{16}\)
Vì \(14< 16\) nên \(\dfrac{1}{14}>\dfrac{1}{16}\)
\(\Rightarrow1-\dfrac{3}{14}< 1-\dfrac{3}{16}\)
Vậy \(\dfrac{11}{14}< \dfrac{13}{16}\)

b)
Ta có:
\(\dfrac{2019}{2020}=1-\dfrac{1}{2020}\)
\(\dfrac{2020}{2021}=1-\dfrac{1}{2021}\)
So sánh phân số \(\dfrac{1}{2020}\) và \(\dfrac{1}{2021}\)
Vì \(2020< 2021\) nên \(\dfrac{1}{2020}>\dfrac{1}{2021}\)
\(\Rightarrow1-\dfrac{1}{2020}< 1-\dfrac{1}{2021}\)
Vậy \(\dfrac{2019}{2020}< \dfrac{2020}{2021}\)

c)
Ta có:
\(\dfrac{205}{207}=1-\dfrac{2}{207}\)
\(\dfrac{2019}{2021}=1-\dfrac{2}{2021}\)
So sánh phân số \(\dfrac{2}{207}\) và \(\dfrac{2}{2021}\)
Vì \(207< 2021\) nên \(\dfrac{1}{207}>\dfrac{1}{2021}\)
\(\Rightarrow1-\dfrac{2}{207}< 1-\dfrac{2}{2021}\)
Vậy \(\dfrac{205}{207}< \dfrac{2019}{2021}\)

d)
Ta có:
\(\dfrac{99}{100}=1-\dfrac{1}{100}\)
\(\dfrac{97}{98}=1-\dfrac{1}{98}\)
So sánh phân số \(\dfrac{1}{100}\) và \(\dfrac{1}{98}\)
Vì \(100>98\) nên \(\dfrac{1}{100}< \dfrac{1}{98}\)
\(\Rightarrow1-\dfrac{1}{100}>1-\dfrac{1}{98}\)
Vậy \(\dfrac{99}{100}>\dfrac{97}{98}\)