[tex]The\ area\ of\ the\ square:A_{\fbox{}}=x^2\\\\The\ area\ of\ the\ triangle:A_\Delta=\frac{1}{2}\cdot\frac{3}{4}x\cdot x=\frac{3}{8}x^2\\\\The\ area\ of\ the\ shaded\ place:\\\\A=A_{\fbox{}}-A_\Delta=x^2-\frac{3}{8}x^2=\frac{8}{8}x^2-\frac{3}{8}x^2=\frac{8-3}{8}x^2=\frac{5}{8}x^2\\\\The\ fraction\ of\ the\ square\ that\ is\ shaded:\frac{A}{A_{\fbox{}}}=\frac{\frac{5}{8}x^2}{x^2}=\boxed{\boxed{\frac{5}{8}}}[/tex]
