In the applet below, please do the following: [br][br]1) Use the PARALLEL LINE [icon]/images/ggb/toolbar/mode_parallel.png[/icon] tool to construct a line through [i]C[/i] parallel to line [i]f[/i]. [br]2) Use the SLOPE [icon]/images/ggb/toolbar/mode_slope.png[/icon] tool to measure and display the slopes of both lines. (They should be equal.) [br]3) If you want to see a grid, right-click and select "Show Grid" (or select the Grid option in upper right). [br][br]4) Use the PERPENDICULAR LINE [icon]/images/ggb/toolbar/mode_orthogonal.png[/icon] tool to construct a line through [i]D[/i] that's perpendicular to [i]f[/i]. [br]5) Plot the point of intersection of line [i]f[/i] and the line you just constructed in (4). [br][br]6) Verify the measure of the angle at which line [i]f[/i] and the line you constructed in (4) = [math]90^\circ[/math]. [br][br][color=#0000ff]When you're done (or if you're unsure of something) feel free to check by watching the brief silent screencast below the applet. [/color]