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\(\dfrac{5}{7}\).\(\dfrac{-3}{11}\)-\(\dfrac{5}{7}\).\(\dfrac{8}{11}\)+1\(\dfrac{5}{7}\)
= \(\dfrac{5}{7}\).(\(\dfrac{-3}{11}\) - \(\dfrac{8}{11}\)) + 1\(\dfrac{5}{7}\)
= \(\dfrac{5}{7}\). -1 + 1\(\dfrac{5}{7}\)
= \(\dfrac{-5}{7}\) +1\(\dfrac{5}{7}\)
= -1
22. 28. 52
= ( 22 . 52 ) . 28
= ( 4 . 25 ) .28
= 100 . 28
= 2800
1) A=\(\dfrac{7}{1.3}+\dfrac{7}{3.5}+\dfrac{7}{5.7}+...+\dfrac{7}{99.101}\)=\(\dfrac{7.2}{1.3.2}+\dfrac{7.2}{3.5.2}+\dfrac{7.2}{5.7.2}+...+\dfrac{7.2}{99.101.2}\)=
\(\dfrac{7}{2}\left(\text{}\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{99.101}\right)\)=
\(\dfrac{7}{2}\left(\text{}1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}...+\dfrac{1}{99}-\dfrac{1}{101}\right)\)=
\(\dfrac{7}{2}\left(\text{}1-\dfrac{1}{101}\right)\)=\(\dfrac{7}{2}.\dfrac{100}{101}=\dfrac{350}{101}\)
2) A=\(\dfrac{1}{2}+\dfrac{2}{2.4}+\dfrac{3}{4.7}+\dfrac{4}{7.11}+\dfrac{5}{11.16}\)=\(\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}\)
=\(\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{16}\)=\(1-\dfrac{1}{16}=\dfrac{15}{16}\)
78 :76 + 4.42 - 13
= 72 + 43 - 13
= 49 + 64 - 13
= 113 - 13
= 100
\(a,=-17+91-91+17+2011=\left(-17+17\right)+\left(91-91\right)+2011\)
\(=0+0+2011=2011\)
ai tick ông v