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ta có
A=\(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+....+\frac{2}{19.21}\)
\(=2\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{19.21}\right)\)
\(=2\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)
\(=2\left(\frac{1}{3}-\frac{1}{21}\right)\)
=\(\frac{4}{7}\)
1/1+2+1/1+2+3+1/1+2+3+4+1/1+2+3+4+5
=1/3+1/6+1/10+1/15
lấy mẫu số chung là 30,ta có:
10/30+5/6+3/10+2/30
=20/30=2/3
\(\frac{\left(\frac{3}{15}+\frac{1}{4}+\frac{7}{20}\right)\times\frac{17}{49}}{5\frac{1}{3}+\frac{2}{5}}\)
\(=\frac{\left(\frac{12}{60}+\frac{15}{60}+\frac{21}{60}\right)\times\frac{17}{49}}{\frac{16}{3}\times\frac{2}{5}}\)
\(=\frac{\frac{48}{60}\times\frac{17}{49}}{\frac{80}{15}+\frac{6}{15}}\)
\(=\frac{\frac{816}{2940}}{\frac{86}{15}}\)
\(=\frac{816}{2940}:\frac{86}{15}\)
\(=\frac{816}{2940}\times\frac{15}{86}\)
\(=\frac{68}{245}\times\frac{15}{86}\)
\(=\frac{102}{2107}\)
=17/6+1/2×13/3-9/4:3/5
=17/6+13/6-9/4×5/3
=17/6+13/6-15/4
=68+52-90/24
=30/24
=5/4
\(2\frac{5}{6}+0,5\times4\frac{1}{3}-2\frac{1}{4}:\frac{3}{5}\)
\(=\frac{17}{6}+\frac{1}{2}\times\frac{13}{3}-\frac{9}{4}:\frac{3}{5}\)
\(=\frac{17}{6}+\frac{13}{6}-\frac{9}{4}\times\frac{5}{3}\)
\(=5-\frac{9\times5}{4\times3}\)
\(=5-\frac{3\times5}{4\times1}\)
\(=5-\frac{15}{4}=\frac{20-15}{4}=\frac{5}{4}\)
Biểu thức 1 :
1 4/5 x 5/6 = 9/5 x 5/6 = 45 /30 = 3/2
Biểu thức 2 :
15/16 : 11 /4 = 15/16 x 4/11 = 15/44
Tk cho mk nha ! Kết bạn với mk nhé !
\(6\frac{1}{2}-2\frac{1}{3}\div\frac{7}{15}\)
=\(\frac{13}{2}-\frac{7}{3}\div\frac{7}{15}\)
=\(\frac{13}{2}-5\)
=\(\frac{3}{2}\)
\(=1-\frac{1}{3}+\frac{2}{3.5}+...+\frac{2}{15.17}\)
\(=1-\frac{1}{3}+2.\left(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{15.17}\right)\)
\(=1-\frac{1}{3}+2.\left(\frac{1}{3}-\frac{1}{17}\right)\)
\(=\frac{62}{51}\)
\(\Leftrightarrow A=1-\frac{1}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+....+\frac{2}{13.15}+\frac{2}{15.17}\)
\(\Leftrightarrow A=\frac{2}{1.3}+\frac{2}{3.5}+......+\frac{2}{13.15}+\frac{2}{15.17}\)
\(\Leftrightarrow A=1-\frac{1}{17}=\frac{16}{17}\)