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Intersection in Geometry – Explanation and Examples
Publish Date: 03 Oct, 2023
Table of Content
Geometry is the study of shapes, sizes, and dimensions of objects. A geometric intersection is an important concept in geometry. It is a point, line, or curve where two or more geometric objects overlap. The types of geometric intersections depend on which geometric objects are intersecting.
The concept of intersection in geometry is essential in solving geometric problems and designing objects in the real world. We will explore the concept of Geometric intersection in this article.
What is a geometric Intersection?
The term ‘Intersection’ in geometry refers to the points where two or more geometric shapes meet each other. It helps to determine how geometric objects interact and relate to one another.
The concept of intersection is also invariant under transformations such as translations, rotations, and reflections. This property is essential when designing objects that need to retain their shape and position in different conditions.
The Intersections can be classified into different types based on the number of dimensions of the shapes or lines involved. Some common types of Geometric intersections are given below:
- Line-plane intersection
- Line-sphere intersection
- Intersection of a polyhedron with a line
- Line segment intersection
- Intersection curve
Intersection in Two-Dimensional Space
The intersection of two-dimensional space can occur in various ways depending on the geometric objects involved. Here are some common scenarios and the formulas to find their intersections:
Intersection of Two Lines:
Given two lines in slope-intercept form:
Line 1: y = m1x + b1
Line 2: y = m2x + b2
Set the two equations equal to each other to find the intersection point (x, y):
m1x + b1 = m2x + b2
Solve for x:
x = (b2 - b1)/ (m1 - m2)
Then, plug x into either equation to find y.
The intersection of a Line and a Circle:
Given a line y = mx + b and a circle with center (h, k) and radius r:
Line: y = mx + b
Circle: (x - h) 2 + (y - k) 2 = r2
Substitute the expression for y from the line equation into the circle equation:
(x - h)2 + (mx + b - k)2 = r2
Solve this equation for x, which may give two intersection points.
Plug the x values back into the line equation to find the corresponding y values.
The intersection of Two Circles:
Given two circles with centers (h1, k1) and ((h2, k2) and radii r1 and r2:
Circle 1: (x - h1)2 + (y - k1)2 = r12
Circle 2: (x - h2)2 + (y - k2)2 = r22
To find the intersection points, solve the two equations simultaneously. This may give two, one, or zero intersection points depending on the relative positions of the circles.
Intersection in Three-Dimensional Space
The intersection of objects in three-dimensional space involves solving equations representing those objects to find the common points or regions where they intersect. Here are some common formulas and methods for finding intersections in three dimensions:
Plane-Plane Intersection:
Two planes represented by their normal vectors and a point on the plane can be intersected by finding a line of intersection:
Plane 1: A1x + B1y + C1z + D1 = 0 (normal vector N1 = <A1, B1, C1>)
Plane 2: A2x + B2y + C2z + D2 = 0 (normal vector N2 = <A2, B2, C2>)
Intersection (Line): P = P0 + t × N (where P0 is a point on the line of intersection, N is the cross product of N1 and N2, and t is a parameter)
Line-Plane Intersection:
A line (parametric equation) and a plane can intersect by substituting the line equation into the plane equation:
Line: P = P0 + t × V
Plane: Ax + By + Cz + D = 0
Substitute the line equation into the plane equation:
A(P0x + tVx) + B(P0y + tVy) + C(P0z + tVz) + D = 0
Now, solve for t:
(AVx + BVy + CVz)t + (AP0x + BP0y + CP0z + D) = 0
Now, you can solve for t:
t = -(AP0x + BP0y + CP0z + D) / (AVx + BVy + CVz)
This value of t will give you the parameter value at which the line intersects the plane.
Sphere-Sphere Intersection:
Two spheres with centers (C1 and C2) and radii (r1 and r2) can intersect in various ways:
No Intersection: If the distance between the centers is greater than the sum of the radii (||C1 - C2|| > r1 + r2).
Single Point Intersection: If the distance between the centers is equal to the sum of the radii (||C1 - C2|| = r1 + r2).
Circle Intersection: If one sphere is completely inside the other (||C1 - C2|| < |r1 - r2|).
Solved Examples of intersections in geometry
We will look at some examples to better understand the concept of intersection in geometry.
Example 1:
Find the intersection of the lines y = 2x + 1 and y = -3x + 5.
Solution:
We need to solve their equations simultaneously to find the intersection point of two lines.
2x + 1 = -3x + 5
5x = 4
x = 4/5
Substituting x = 4/5 into either equation, we get:
y = 2(4/5) + 1
= 9/5
∴ the intersection point of the two lines is (4/5, 9/5).
Example 2:
Find the intersection of the circle x2 + y2 = 4 and the line y = 2x - 1.
Solution:
Substitute the equation of the line into the equation of the circle.
x2 + (2x - 1)2 = 4
x2 + 4x2 - 4x + 1 = 4
5x2 - 4x - 3 = 0
Using the quadratic formula:
∴ x = [- b ± √(b2 – 4ac)] / 2a
x = (4 ± √ (76))/10
Substituting x into the equation of the line, we get:
y = (2(4 ± √ (76))/10) – 1
= (4 ± √ (76))/5) – 1
= (4 ± √ (76)) - 5)/5
y = (-1 ± √ (76))/5
∴ The intersection points of the circle and the line are (4 ± √ (76))/10 and (-1 ± √ (76))/5.
Conclusion
We have explored the concept of geometric intersection, which is the point, or set of points where geometric shapes or lines meet or cross. We covered specific cases in two and three-dimensional spaces. We have also solved some examples to illustrate how to find the intersection points of geometric shapes and lines
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