Alternate ways to show a line is a tangent to a curve

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SUMMARY

The discussion centers on proving that the line equation x + 2y = 7 is a tangent to the circle defined by x² + y² - 4x - 1 = 0. The primary method involves demonstrating that there is only one point of intersection between the line and the circle, which confirms tangency. Alternative approaches mentioned include using the relationship between the gradients of perpendicular lines and the distance from the circle's center to the line, which also leads to the conclusion of a single intersection point.

PREREQUISITES
  • Understanding of circle equations and their standard forms
  • Knowledge of linear equations and their graphical representations
  • Familiarity with the concept of tangents in geometry
  • Basic calculus principles, particularly derivatives and gradients
NEXT STEPS
  • Study the method of finding tangents to circles using derivatives
  • Explore the geometric interpretation of gradients and perpendicular lines
  • Learn about the distance from a point to a line in coordinate geometry
  • Investigate the conditions for tangency in conic sections
USEFUL FOR

Students studying geometry, particularly those focusing on conic sections, as well as educators seeking to enhance their teaching methods for tangents and intersections in mathematics.

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Homework Statement


Show that x+2y=7 is a tangent to the circle x^2+y^2-4x-1=0



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The Attempt at a Solution



One way would be to solve simultaneously by substituting for x or y and getting a perfect square showing that there is only one point of intersection. Is there any other way to do this? I was thinking about the idea that the angle made by a tangent and a radius is 90 degrees and that the product of the gradients of perpendicular lines is -1. Are such thoughts correct ones?
 
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With calculus you can show that the given line and the line tangent to the circle at the single intersection point are one and the same. But all you need to do is prove that there is only one intersection
 
Yes they are, however all you are showing is that the straight line is perpendicular to a radius, you arent showing if it actually touches the circle or not, and if it does, how many points.
 
So there is no other way to show it is a tangent than only showing that there is only one point of intersection?
 
You could also show that the distance to the line on a line from the center of the circle that is normal to your line has length equal to the radius of the circle. But that's the same thing as showing there is only one intersection and takes a lot more words to express.
 
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