recall that
1² = 1
1⁴ = 1
1¹⁰⁰⁰⁰⁰⁰⁰⁰⁰ = 1
[tex]\bf \cfrac{d^2-1}{d^2-d}\div \cfrac{d+1}{d-1}\implies \cfrac{d^2-1}{d^2-d}\cdot \cfrac{d-1}{d+1}\implies \cfrac{\stackrel{\stackrel{\textit{difference of}}{\textit{squares}}}{d^2-1^2}}{d(d-1)}\cdot \cfrac{d-1}{d+1} \\\\\\ \cfrac{\begin{matrix} (d+1) (d-1) \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}}{d~~\begin{matrix} (d-1) \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}}\cdot \cfrac{d-1}{\begin{matrix} d+1 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix} }\implies \cfrac{d-1}{d}[/tex]
