Tính và so sánh : 1. A= 1/2 x 2/3 + 3/10 và B= 7/8 - 1/4 x 3/2 2. C= 2/3 x 4/5 + 8/15 - 1/15 và D= 1/2 x 5/6 + 2/3 x 3/4 3. E= 3/10 x -7/10 + -1/2 x -7/50 + 1/4 x 3/25 - 1/100 và F= -3/2 x -7/10 + 1/4 x 7/5 + -1/20

1)

→ Ta có :

A = $\dfrac{1}{2}$ . $\dfrac{2}{3}$ + $\dfrac{3}{10}$

⇒ A = $\dfrac{2}{6}$ + $\dfrac{3}{10}$

⇒ A = $\dfrac{1}{3}$ + $\dfrac{3}{10}$

⇒ A = $\dfrac{10 + 9}{30}$

⇒ A = $\dfrac{19}{30}$

→ Ta lại có :

B = $\dfrac{7}{8}$ - $\dfrac{1}{4}$ . $\dfrac{3}{2}$

⇒ B = $\dfrac{7}{8}$ - $\dfrac{3}{8}$

⇒ B = $\dfrac{4}{8}$

⇒ B = $\dfrac{1}{2}$

→ Ta dễ dàng thấy A > B.

2)

→ Ta có :

C = $\dfrac{2}{3}$ . $\dfrac{4}{5}$ + $\dfrac{8}{15}$ - $\dfrac{1}{15}$

⇒ C = $\dfrac{8}{15}$ + $\dfrac{7}{15}$

⇒ C = $\dfrac{15}{15}$

⇒ C = 1

→ Ta lại có :

D = $\dfrac{1}{2}$ . $\dfrac{5}{6}$ + $\dfrac{2}{3}$ . $\dfrac{3}{4}$

⇒ D = $\dfrac{5}{12}$ + $\dfrac{6}{12}$

⇒ D = $\dfrac{11}{12}$

→ Ta dễ dàng thấy C > D.

3)

→ Ta có :

E = $\dfrac{3}{10}$ . ( - $\dfrac{7}{10}$ ) - $\dfrac{1}{2}$ . ( - $\dfrac{7}{50}$ ) + $\dfrac{1}{4}$ .

$\dfrac{3}{25}$ - $\dfrac{1}{100}$

⇒ E = $\dfrac{- 21}{100}$ + $\dfrac{7}{100}$ + $\dfrac{3}{100}$ - $\dfrac{1}{100}$

⇒ E = $\dfrac{- 21 + 7 + 3 - 1}{100}$ = $\dfrac{- 3}{25}$

→ Ta lại có :

F = - $\dfrac{3}{2}$ . ( - $\dfrac{7}{10}$ ) + $\dfrac{1}{4}$ . $\dfrac{7}{5}$ - $\dfrac{1}{20}$

⇒ F = $\dfrac{21}{20}$ + $\dfrac{7}{20}$ - $\dfrac{1}{20}$

⇒ F = $\dfrac{27}{20}$

→ Ta dễ dàng thấy F > E.

5 sao nha