\(\dfrac{3}{5}=\dfrac{3}{5}\)

\(\dfrac{33}{55}=\dfrac{33:11}{55:11}=\dfrac{3}{5}\)

\(\Rightarrow\dfrac{3}{5}=\dfrac{33}{55}\)

\(\dfrac{7}{x-1}=\dfrac{5}{x-4}\left(x\ne1;4\right)\)

\(\Leftrightarrow\dfrac{7\left(x-4\right)}{\left(x-1\right)\left(x-4\right)}=\dfrac{5\left(x-1\right)}{\left(x-1\right)\left(x-4\right)}\)

\(\Leftrightarrow7x-28=5x-5\)

\(\Leftrightarrow7x-5x=-5+28\)

\(\Leftrightarrow2x=23\)

\(\Leftrightarrow x=\dfrac{23}{2}\left(Thoaman\right)\)

Thể tích của hình đó là : 

\(75\times40\times35=105000\left(cm^3\right)\)

\(\dfrac{3}{x-4}-\dfrac{4}{4-x}=\dfrac{3x-4}{x^2-16}\left(Dk:x\ne\pm4\right)\)

\(\Leftrightarrow\dfrac{3}{x-4}-\dfrac{4}{x+4}=\dfrac{3x-4}{\left(x-4\right)\left(x+4\right)}\)

\(\Leftrightarrow\dfrac{3\left(x+4\right)}{\left(x-4\right)\left(x+4\right)}-\dfrac{4\left(x-4\right)}{\left(x-4\right)\left(x+4\right)}=\dfrac{3x-4}{\left(x-4\right)\left(x+4\right)}\)

\(\Leftrightarrow3x+12-4x+16=3x-4\)

\(\Leftrightarrow3x-4x-3x=-4-16-12\)

\(\Leftrightarrow-4x=-32\)

\(\Leftrightarrow x=8\left(Thoaman\right)\)