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\(=\dfrac{3}{2}-\dfrac{2}{21}-\dfrac{7}{12}+\left[\dfrac{15}{21}-\dfrac{1}{3}+\dfrac{5}{4}-\dfrac{2}{7}-\dfrac{1}{3}\right]\)
=11/12-2/21+5/7-2/3+5/4-2/7
=11/12-2/3+5/4-2/21+3/7
=11/12-8/12+15/12-2/21+9/21
=18/12+7/21
=3/2+1/3
=9/6+2/6=11/6
16:Sửa đề: \(\frac12x+\frac16\left(x-2\right)=\frac34-2x\)
=>\(\frac12x+\frac16x-\frac26=\frac34-2x\)
=>\(\frac46x-\frac13=\frac34-2x\)
=>\(\frac23x+2x=\frac34+\frac13\)
=>\(\frac83x=\frac{9}{12}+\frac{4}{12}=\frac{13}{12}\)
=>\(x=\frac{13}{12}:\frac83=\frac{13}{12}\times\frac38=\frac{13}{4\times8}=\frac{13}{32}\)
19: \(\frac{5}{12}x+3=\frac13-\frac{7}{12}x\)
=>\(\frac{5}{12}x+\frac{7}{12}x=\frac13-3\)
=>\(\frac{12}{12}x=\frac13-\frac93=-\frac83\)
=>\(x=-\frac83\)
20: \(\frac12x+\frac52=\frac72x-\frac34\)
=>\(\frac12x-\frac72x=-\frac34-\frac52\)
=>\(-3x=-\frac34-\frac{10}{4}=-\frac{13}{4}\)
=>\(3x=\frac{13}{4}\)
=>\(x=\frac{13}{4}:3=\frac{13}{12}\)
a) \(\left(\frac{3}{5}x-\frac{2}{3}x-x\right).\frac{1}{7}=\frac{-5}{21}\)
\(\Rightarrow\left(\frac{3}{5}-\frac{2}{3}-1\right).x=\frac{-5}{21}:\frac{1}{7}=\frac{-5}{3}\)
\(\Rightarrow\frac{-16}{15}.x=\frac{-5}{3}\Rightarrow x=\frac{-5}{3}:\frac{-16}{15}=\frac{25}{16}\)
b) \(\left(x-\frac{1}{4}\right)^2=\frac{1}{36}\)
\(\Rightarrow\left(x-\frac{1}{4}\right)^2=\left(±\frac{1}{6}\right)^2\)
\(\Rightarrow\orbr{\begin{cases}x-\frac{1}{4}=\frac{1}{6}\\x-\frac{1}{4}=\frac{-1}{6}\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}x=\frac{5}{12}\\x=\frac{1}{12}\end{cases}}\)
c/
C = 1/100-1/100-1/99-1/99-1/98-1/98-1/97-..........-1/3-1/2-1/2-1/1
C = 1/100-1/100-1/1
C = 0-1/1
C = -1
\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left\{\dfrac{7}{12}-\left[\dfrac{15}{21}-\left(\dfrac{1}{3}-\dfrac{5}{4}\right)-\left(\dfrac{2}{7}+\dfrac{1}{3}\right)\right]\right\}\)
\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left\{\dfrac{7}{12}-\dfrac{85}{84}\right\}\)
\(B=\dfrac{3}{2}-\dfrac{2}{21}-\left(-\dfrac{3}{7}\right)\)
\(B=\dfrac{11}{6}\)