Identity for tan(x-y) = [tan(x) - tan(y)]/[1-tan(x)tan(y)]

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Discussion Overview

The discussion revolves around the identity for tan(x-y) and its application in determining the tangent of the angle between two intersecting lines based on their slopes. It includes elements of mathematical reasoning and clarification of concepts related to slopes and angles.

Discussion Character

  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about how to apply the identity for tan(x-y) to the problem involving the slopes of two intersecting lines.
  • Another participant clarifies that the slope of a line is the tangent of the angle it makes with the x-axis, emphasizing the relevance of this definition to the exercise.
  • A suggestion is made to draw a diagram to visualize the problem, noting that shifting the coordinate system does not affect the angles or slopes involved.
  • It is pointed out that if the angles made by the lines with the x-axis are θ1 and θ2, the angle between the lines can be determined, and the tangent of that angle can be calculated.
  • One participant acknowledges a typo in their earlier message, indicating a light-hearted acknowledgment of the clarification provided by another participant.

Areas of Agreement / Disagreement

Participants generally agree on the definition of the slope of a line as the tangent of its inclination, but there is some initial confusion regarding the application of the identity to the problem at hand.

Contextual Notes

The discussion does not resolve the initial confusion expressed by the first participant, and the application of the identity remains an open question.

BrendanM
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Use the identity for tan(x-y) = [tan(x) - tan(y)]/[1-tan(x)tan(y)] to show that if two lines L1 and L2 intersect at angle theta then tan(theta) = m2-m1/(1 + m1m2) where m1 and m2 are the slopes of L1 and L2 respectivly.

hmm ihave no idea where to start for this it doesn't make sense to me. please help
 
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Hint: The slope of a straight line is the angle the line makes with the x-axis.
 
No, the slope of a line is the TANGENT of the angle the line makes with the x-axis. Which is the whole point of this exercise!

1) Draw a picture. You can always shift you coordinate system up or down, right or left without changing angles (or slopes) so draw it so that the two lines intersect at the origin. If the angles the two lines make with the x-axis are θ1 and θ2, what is the angle between them? What is the tangent of that angle?
 
HallsofIvy is correct. The slope of a line is the tangent of its inclination.
 
I knew that! Thanks for pointing out my typo! :-)
 

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