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\(\frac{3}{3.6.9}+\frac{3}{6.9.12}+\frac{3}{9.12.15}+\frac{3}{12.15.18}=\frac{3}{6}\left(\frac{6}{3.6.9}+\frac{6}{6.9.12}+\frac{6}{9.12.15}+\frac{6}{12.15.18}\right)\)
= \(\frac{1}{2}\left(\frac{1}{3.6}-\frac{1}{6.9}+\frac{1}{6.9}-\frac{1}{9.12}+\frac{1}{9.12}-\frac{1}{12.15}+\frac{1}{12.15}-\frac{1}{15.18}\right)\)
\(=\frac{1}{2}\left(\frac{1}{3.6}-\frac{1}{15.18}\right)=\frac{1}{2}.\frac{14}{270}=\frac{7}{270}\)
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\(2x+\dfrac{1}{2}+4x+\dfrac{1}{6}+6x+\dfrac{1}{6}=12\dfrac{11}{12}\Leftrightarrow\)\(2x+\dfrac{1}{2}+4x+\dfrac{1}{6}+6x+\dfrac{1}{6}=\dfrac{155}{12}\)\(\Leftrightarrow24x+6+48x+2+72x+2=155\)\(\Leftrightarrow24x+48x+72x=155-6-2-2\)\(\Leftrightarrow144x=145\)\(\Leftrightarrow x=\dfrac{145}{144}\)
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-12.6=-6.12=-72
-12 x 6 = -72
-6 x 12 = -72