Chọn B

\(\dfrac{4}{7}+\dfrac{11}{21}=\dfrac{4\cdot3+11}{21}=\dfrac{23}{21}\)

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\(=25\%.\frac{\left(-21\right)}{4}\)

\(=\frac{1}{4}.\frac{\left(-21\right)}{4}\)

\(=-\frac{21}{16}\)

25% của \(-\frac{21}{4}\) là: \(-\frac{21}{4}.25\%=-\frac{21}{4}.\frac{1}{4}=-\frac{21}{16}\)

Vậy 25% của \(-\frac{21}{4}\) là \(-\frac{21}{16}\)

25%của -21/4 là =-21/4.1/4 =-21/16

\(\frac{-25}{100}\)của \(\frac{-21}{4}\)là: \(\frac{-25}{100}\times\frac{-21}{4}=\)\(\frac{21}{16}\)

`21/25-5/21-[-4]/25-16/21+1/2`

`=(21/25-[-4]/25)-(5/21+16/21)+1/2`

`=25/25-21/21+1/2`

`=1-1+1/2=1/2`

\(1\frac{4}{25}+\frac{7}{21}-\frac{4}{25}+0,5+\frac{14}{21}\)

\(=\left(1\frac{4}{25}-\frac{4}{25}\right)+\left(\frac{7}{21}+\frac{14}{21}\right)+0,5\)

\(=1+0+0,5\)

\(=1,5\)

\(15\frac{1}{4}:\frac{-5}{8}-25\frac{1}{4}:\frac{-5}{8}\)

\(=-\frac{8}{5}.\left(15\frac{1}{4}-25\frac{1}{4}\right)\)

\(=-\frac{8}{5}.\left(-10\right)\)

\(=16\)

a) \(\dfrac{7}{-25}+\dfrac{8}{25}=\dfrac{-7}{25}+\dfrac{8}{25}=\dfrac{1}{25}\)

\(\dfrac{4}{5}+\dfrac{4}{-18}=\dfrac{4}{5+\left(-18\right)}=\dfrac{4}{-13}=\dfrac{-4}{13}\)

\(\dfrac{7}{21}+\dfrac{9}{-36}=\dfrac{1}{3}+\dfrac{1}{-4}=\dfrac{1}{3+\left(-4\right)}=\dfrac{1}{-1}=1\)

b) \(\dfrac{12}{20}-\dfrac{2}{5}=\dfrac{12}{20}+\left(\dfrac{-2}{5}\right)=\dfrac{3}{5}+\dfrac{-2}{5}=\dfrac{3+\left(-2\right)}{5}=\dfrac{1}{5}\)

\(\dfrac{2}{3}-\dfrac{-5}{6}=\dfrac{2}{3}+\dfrac{5}{6}=\dfrac{4}{6}+\dfrac{5}{6}=\dfrac{9}{6}=\dfrac{3}{2}\)

\(6-\dfrac{3}{20}=6+\dfrac{-3}{20}=\dfrac{60}{60}+\dfrac{-9}{60}=\dfrac{51}{60}\)

c) \(\dfrac{2}{21}.\dfrac{-7}{3}=\dfrac{2.\left(-1\right)}{7.3}=\dfrac{-2}{21}\)

\(\dfrac{27}{28}.\left(-21\right)=\dfrac{27.\left(-21\right)}{28}=\dfrac{-567}{28}\)

\(\dfrac{-15}{7}.\dfrac{-14}{25}=\dfrac{-3.\left(-2\right)}{1.5}=\dfrac{6}{5}\)

(- 4) . (+21) . (- 25) . (- 2)

= (-4).(-25).(+21).(-2)

= 100.(+21).(-2)

= -4200

Đáp án: C

2/14+3/21=1/7+1/7=2/7

4/5+15/25=4/5+3/5=7/5

21/27+4/9=7/9+4/9=11/9