b:
\(\dfrac{-4}{7}=\dfrac{x-40}{31-5x}\)
=>\(\dfrac{x-40}{5x-31}=\dfrac{4}{7}\)
=>7x-280=20x-124
=>-13x=156
=>x=-12
d: =>Sửa đề: \(\dfrac{1}{21}+\dfrac{1}{28}+\dfrac{1}{36}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2}{9}\)
=>\(\dfrac{2}{42}+\dfrac{2}{56}+\dfrac{2}{72}+...+\dfrac{2}{\left(x+1\right)\cdot x}=\dfrac{2}{9}\)
=>1/42+1/56+...+1/x(x+1)=1/9
=>1/6-1/7+1/7-1/8+...+1/x-1/x+1=1/9
=>1/6-1/(x+1)=1/9
=>1/x+1=1/18
=>x+1=18
=>x=17
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