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Lời giải:
$=\frac{21\times 2\times 12-42\times 12}{15+38:19}+16:4+16$
$=\frac{42\times 12-42\times 12}{15+38:19}+4+16$
$=\frac{0}{15+38:9}+4+16=0+4+16=20$
\(\dfrac{1}{10}+\dfrac{2}{10}+\dfrac{3}{10}+\dfrac{4}{10}+\dfrac{5}{10}+\dfrac{6}{10}+\dfrac{7}{10}+\dfrac{8}{10}+\dfrac{9}{10}\)
\(=\left(\dfrac{1}{10}+\dfrac{9}{10}\right)+\left(\dfrac{2}{10}+\dfrac{8}{10}\right)+\left(\dfrac{3}{10}+\dfrac{7}{10}\right)+\left(\dfrac{4}{10}+\dfrac{6}{10}\right)+\dfrac{5}{10}\)
\(=1+1+1+1+\dfrac{5}{10}\)
\(=4+\dfrac{5}{10}\)
\(=\dfrac{45}{10}\)
\(13,25:0,5+13,25:0,25+13,25:0,125+13,25\times6\)
\(=13,25:\dfrac{1}{2}+13,25:\dfrac{1}{4}+13,25:\dfrac{1}{8}+13,25\times6\)
\(=13,25\times2+13,25\times4+13,25\times8+13,25\times6\)
\(=13,25\times\left(2+4+8+6\right)\)
\(=13,25\times20\)
\(=265\)
Tính bằng cách thuận tiện
\(\dfrac{7}{13}\) + \(\dfrac{5}{9}\) + \(\dfrac{6}{13}\) + \(\dfrac{5}{9}\)
`7/13+5/9+6/13+5/9`
`=(7/13+6/13)+(5/9+5/9)`
`=13/13+10/9`
`=1+10/9`
`=9/9+10/9`
`=19/9`
Bài 3
a,26/100+0,009+41/100+0,24
0,26+0,09+0,41+0,24
(0,26+0,24)+(0,09+0,41)
0,5+0,5
=1
b,9+1/4+6+2/7+7+3/5+8+2/3+2/5+1/3+5/7+3/4
(9+6+7+8)+(2/7+5/7)+(1/4+3/4)+(3/5+2/5)+(2/3+1/3)
30+1+1+1+1
=34
Bài 4,5 khó quá mik ko bít lamf^^))
Bài 4: a, \(\dfrac{2008}{2009}\) < 1; \(\dfrac{10}{9}\) > 1
\(\dfrac{2008}{2009}\) < \(\dfrac{10}{9}\)
b, \(\dfrac{1}{a+1}\) và \(\dfrac{1}{a-1}\)
Ta có: a + 1 > a - 1 ⇒ \(\dfrac{1}{a+1}\) < \(\dfrac{1}{a-1}\)
\(=\dfrac{12\times101\times169\times1001}{13\times101\times144\times1001}=\dfrac{13}{12}\)
\(\dfrac{7}{6}\times\dfrac{3}{4}+\dfrac{3}{4}\times\dfrac{5}{6}-\dfrac{3}{4}\\ =\left(\dfrac{7}{6}+\dfrac{5}{6}-1\right)\times\dfrac{3}{4}\\ =1\times\dfrac{3}{4}=\dfrac{3}{4}\\ \dfrac{6\times7\times15\times8}{4\times21\times10\times3}\\ =\dfrac{2\times3\times7\times5\times3\times4\times2}{4\times7\times3\times5\times2\times3}\\ =2\)
\(\dfrac{9}{16}\) : \(\dfrac{8}{27}\) :\(\dfrac{6}{19}\) = \(\dfrac{9}{16}\) x \(\dfrac{27}{8}\) x \(\dfrac{19}{6}\) =(\(\dfrac{9}{16}\)x\(\dfrac{16}{9}\)) x \(\dfrac{27}{8}\)
= 1 x\(\dfrac{27}{8}\)=\(\dfrac{27}{8}\)
Thank you ^^