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\Huge\sf Point-slope formula for a straight line (can you see why it works?)
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\Large\sf
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\psplot[linecolor=red, border=1pt]{-2.45}{10.45}{x/2-1}
\uput[ul](4,1){$(a,b)$}
\psline(4,1)(8,1)(8,3)
\uput[ul](8,3){$(x,y)$}
\uput[d](6,1){$x-a$}
\uput[r](8,2){$y-b$}
\uput[r](.5,3.5){\begin{tabular}{l}
Given a point $(a,b)$ and a number $m$\\[5pt]
consider all $(x,y)$ such that \\[5pt]
slope \ $\displaystyle \frac{y-b}{x-a}=m$\\[17pt]
\LARGE\color{red}$\Rightarrow \quad y-b = m(x-a)$
\end{tabular}}
\uput[r](.5,-1.5){Solving for $y$ we get $y = mx+(-ma+b)$ in standard form}
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\begin{center}\tiny 
Point-slope formula for a straight line --- 
Math Poster 2007 --- math.chapman.edu
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