Derivation of linear interpolation? Anyone know a good derivation of the linear interpolation:

$$\frac{y-y_0}{x-x_0}=\frac{y_1-y_0}{x_1-x_0}$$

Wikipedia gives one, which I don't understand.
Since it is a linear interpolation, just consider a straight line $y=a+ bx$ which goes through two points $(x_0,y_0)$ and $(x_1,y_1)$. So $$y_0=a+b x_0$$ $$y_1=a+b x_1$$ Solve for $a,b$.
For any two points: $(x_0 ,y_0 )$ and $(x_1 ,y_1 )$

$y_0 =mx_0 +b$  and $y_1 =mx_1 +b$

Solve for $b$ for each equation...

Then set the two equal to each other...

Next solve for $m$

you get: $m=(y_0 -y_1 )/(x_0 -x_1 )$

Since this is true for any two points on the line, its also true for the first point and another point on the line where you may know the $y$ but not the $x$:
say $(x,y_2 )$

so then:
$$m= \frac{y_0 -y_1 }{x_0 -x_1 }=\frac{y_0 -y_2 }{x_0 -x}.$$

There it is!