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### Course: Geometry (FL B.E.S.T.)>Unit 8

Lesson 10: Constructing a line tangent to a circle

# Geometric constructions: circle tangent

Sal constructs a line tangent to a circle using compass and straightedge.

## Want to join the conversation?

• Is there anything that relates tan line to tan in trigonometry
• Yes, Kevin, there is a relationship. The length of a line segment that is tangent to a circle AND that connects to a central angle of that circle is the tangent of the central angle.
• Is there an exercise like the one your doing?
• How do you use a compass correctly in real life? Every time I do it, it slides and I can't keep it straight. Any tips?
• I would suggest getting a metal compass and pressing down very hard into the paper where you are drawing. Plastic compasses can slide very easily.
• Something wrong with the order of videos. This should be the first one in the constructing playlist not the last
• Shouldn't the line be on the black dot?
• It is! :D
The line that is coming out of the radius is not the tangent line, it is just a straightedge used to help Sal actually find the tangent, so you can ignore that and look at the line perpendicular to it.
In summary, Sal is just trying to demonstrate how to get the most accurate figure of a tangent line.
I hope this helps. :)
(1 vote)
• Why do the subtitles have <i>P</i> when he says P?
• how do you draw two tangents to a circle of radius 4cm so that the angle between them is 45degrees??
(1 vote)
• Since tangents are always at 90 degrees to a radius drawn to it, draw any line segment from the center to beyond the line. From any point on the line, you could draw two tangents, So since you want 45 degrees between them, If you divide this in half, you get 22.5 degrees. This would mean you have a right triangle, so find the complement by 90 - 22.5 = 67.5 degrees. So using a protractor, draw an angle with the center as a vertex that is 67.5 degrees, repeat this on the other side of the line you originally drew, and draw a radius using these degrees. This will create the 45 degree angle.
• What are the parametric equations of a circle??
• Parametric equations are a way to show how the progression of a certain scenario, like how a ball travels as it falls from a certain height in a parabola. For the circle specifically, given the equation the circle in standard form, you will have to use trigonometry, which you may or may not have learned.
• explain pi in simple terms
(1 vote)
• π is a number, just like 4 or -17 or √2. It is a little more than 3. If you divide the circumference of a circle by the length of its radius, the result will be π, for any circle.