Find Direct Common Tangent of 2 Circles without Complexity

In summary, the formula for calculating the direct common tangent of two circles can be obtained by shrinking or enlarging one of the circles and drawing a tangent from the center of the smaller circle to the new circle. The angle theta can be determined using the distance between the centers and the difference of the radii. The equation of the green lines can then be used to find the tangent points on the circles.
  • #1
Dumbledore211
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Homework Statement



Is there any direct formula for calculating the direct common tangent of two circles without having to go all the trouble of using y-y1=m(x1-x2) to derive it for two separate tangents t1 and t2. If there is could anyone explain to me how it is derived?

Homework Equations





The Attempt at a Solution

 
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  • #2
How would you draw those common tangents?

The problem can be reduced to the problem of drawing tangent from a point. Shrink (or blow up) the radius of the bigger circle by the radius of the smaller one. The tangent to the new circle from the centre of the smaller circle e is parallel to tangent line t.
It is easy to get angle theta from the distance of the centres (d) and the difference of the radii. Write up the equation of the green lines. They intersect the tangent points on the circles.

ehild
 

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What is a direct common tangent of two circles?

A direct common tangent of two circles is a straight line that touches both circles at exactly one point. This means that the tangent line is perpendicular to the radius of both circles at the point of tangency.

Why is it important to find the direct common tangent of two circles?

Finding the direct common tangent of two circles is important in various fields, such as geometry, engineering, and physics. It helps with determining the intersection points of two circles, which is useful in solving problems involving circles, such as calculating distances and angles.

What is the mathematical formula for finding the direct common tangent of two circles?

The formula for finding the direct common tangent of two circles is (x-x1)(x-x2)+(y-y1)(y-y2)=r1^2-r2^2, where (x1,y1) and (x2,y2) are the coordinates of the centers of the circles, and r1 and r2 are the radii of the circles.

Is it possible for two circles to have more than one direct common tangent?

Yes, it is possible for two circles to have two direct common tangents if the circles are internally tangent (one circle is inside the other) or externally tangent (circles do not intersect but are close to each other).

What is the difference between a direct common tangent and a common tangent?

A direct common tangent is a straight line that touches both circles at exactly one point, while a common tangent is a line that intersects both circles at two distinct points. Therefore, a direct common tangent is a special case of a common tangent.

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