`a,` `{x-1}/20={y+2}/15=2/5`
$⇒\begin{cases}\dfrac{x-1}{20}=\dfrac{2}{5}\\\dfrac{y+2}{15}=\dfrac{2}{5}\end{cases}$
$⇒\begin{cases}5(x-1)=2.20\\\\5(y+2)=2.15\end{cases}$
$⇒\begin{cases}5x-5=40\\\\5y+10=30\end{cases}$
$⇒\begin{cases}5x=45\\\\5y=20\end{cases}$
$⇒\begin{cases}x=9\\\\y=4\end{cases}$
`b,` `4x-(x+2)=20-(x-2)`
`⇒4x-x-2=20-x+2`
`⇒4x-x+x=20+2+2`
`⇒4x=24`
`⇒x=6`
`c,` `48-3(x-1)^2=(-6)^2`
`⇒3(x-1)^2=12`
`⇒(x-1)^2=4`
`⇒(x-1)^2=(±2)^2`
`⇒x-1=±2`
\(⇒\left[ \begin{array}{l}x-1=2\\x-1=-2\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=3\\x=-1\end{array} \right.\)
`d,` `\frac{x+2}{4}=\frac{x-7}{5}`
`⇒5(x+2)=4(x-7)`
`⇒5x+10=4x-28`
`⇒5x-4x=-28-10`
`⇒x=-38`
