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This interactive geogebra applet allows you to create a line of best fit visually by attempting to minimize the total areas of the squares displayed.

The purpose of this task is to use similar triangles in order to study the coordinates of points which divide a line segment in a given ratio. These coordinates can be calculated directly but the method employed here not only allows us to find the coordinates of the desired points but also to construct them (with straightedge and compass).

The goal of this task is to prove the slope criterion for parallel lines. There are several subtleties to address, including:
separating out the case of vertical lines which do not have well-defined slope,
establishing both implications (parallel lines have equal slope and lines of equal slope are parallel),
relating the algebra of the linear equations to the geometry of lines.

The goal of this task is to apply dilations to a triangle in the coordinate plane. 

The goal of this task is to use similar triangles to establish the slope criterion for perpendicular lines. Students need to be familiar with scaling and with the side-side-side congruence criterion.

The goal of this task is to examine when two lines in the plane are perpendicular in terms of their slopes.