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Bài 1:
\(\frac{3}{5}+\frac{4}{15}=\frac{9}{15}+\frac{4}{15}=\frac{13}{15}\)
\(\frac{5}{6}:\frac{-7}{12}=\frac{5}{6}.\frac{-12}{7}=\frac{-60}{42}=\frac{-10}{7}\)
\(\frac{-21}{24}:\frac{-14}{8}=\frac{-21}{24}.\frac{-8}{14}=\frac{168}{336}=\frac{1}{2}\)
\(\frac{4}{5}:\frac{-8}{15}=\frac{4}{5}.\frac{-15}{8}=\frac{-60}{40}=\frac{-3}{2}\)
\(\frac{5}{12}-\frac{-7}{6}=\frac{5}{12}+\frac{7}{6}=\frac{5}{12}+\frac{14}{12}=\frac{19}{12}\)
\(\frac{-15}{16}.\frac{8}{25}=\frac{-120}{400}=\frac{-3}{10}\)
Bài 2 :
\(6\frac{4}{5}-\left(1\frac{2}{3}+3\frac{4}{5}\right)\)
\(=\frac{34}{5}-\left(\frac{5}{3}+\frac{19}{5}\right)\)
\(=\frac{34}{5}-\frac{5}{3}-\frac{19}{5}\)
\(=\left(\frac{34}{5}-\frac{19}{5}\right)-\frac{5}{3}\)
\(=3-\frac{5}{3}\)
\(=\frac{4}{3}\)
\(6\frac{5}{7}-\left(1\frac{2}{3}+2\frac{5}{7}\right)\)
\(=\frac{47}{7}-\left(\frac{5}{3}+\frac{19}{7}\right)\)
\(=\frac{47}{7}-\frac{5}{3}-\frac{19}{7}\)
\(=\left(\frac{47}{7}-\frac{19}{7}\right)-\frac{5}{3}\)
\(=4-\frac{5}{3}\)
\(=\frac{7}{3}\)
\(\frac{4}{19}.\frac{-3}{7}+\frac{-3}{7}.\frac{15}{19}+\frac{5}{7}\)
\(=\left(\frac{4}{19}+\frac{15}{19}\right).\frac{-3}{7}+\frac{5}{7}\)
\(=1.\frac{-3}{7}+\frac{5}{7}\)
\(=\frac{-3}{7}+\frac{5}{7}\)
\(=\frac{2}{7}\)
\(\frac{5}{9}.\frac{7}{13}+\frac{5}{9}.\frac{9}{13}-\frac{5}{9}.\frac{3}{13}\)
\(=\frac{5}{9}.\left(\frac{7}{13}+\frac{9}{13}-\frac{3}{13}\right)\)
\(=\frac{5}{9}.1\)
\(=\frac{5}{9}\)
a) A = { a \(\in\) N | 10 < a < 17}
b) B = { b \(\in\) N | 0 < b \(\le\) 23}
cau 2:So phan tu cua tap hop A la: (80-4):2+1=39 (phan tu) cau 3:so phan tu cua cac so tu nhien khong vuot qua 20 la: (20-0):1+1=21(phan tu) cau 4:Q={20;31;42;53;64;75;86;97},co 8 phan tu cau 5:co the lap duoc 12 so cau 6:so phan tu co the nhieu nhat cua C la 2 cau 7: co 3 tap hop con cua M gom nhung so chan cau 8:{0;6;12;18;24},co 5 phan tu cau 9: goi so can tim la :abc(a khac 0;a,b,c la chu so ) theo bai ra ,ta co: 1abc=abc .9 1000+abc =abc .9 1000=abc .8 => abc=125 cau 10 :so tap hop con gom 2 phan tu la:6
Ta có : 1/4=1/2*2>1/2*3
1/9=1/3*3>1/3*4
...
1/100=1/10*10>1/10*11
=>A>1/2*3+1/3*4+...+1/10*11=1/2 - 1/3+1/3 - 1/4 +...+1/10 - 1/11
=1/2 - 1/11=9/22=54/132<65/132(bạn hình như viết sai đầu bài chứ cách này đúng mà!)
cảm ơn nha