Combinations of Lines & Planes (Edexcel A Level Further Maths: Core Pure)

How do I tell if a line is parallel to a plane?

• A line is parallel to a plane if its direction vector is perpendicular to the plane’s normal vector
• If you know the Cartesian equation of the plane in the form  then the values of a, b, and c are the individual components of a normal vector to the plane
• The scalar product can be used to check in the direction vector and the normal vector are perpendicular
  • If two vectors are perpendicular their scalar product will be zero

 

How do I tell if the line lies inside the plane?

• If the line is parallel to the plane then it will either never intersect or it will lie inside the plane
  • Check to see if they have a common point
• If a line is parallel to a plane and they share any point, then the line lies inside the plane

 

How do I find the point of intersection of a line and a plane in Cartesian form?

• If a line is not parallel to a plane it will intersect it at a single point
• If both the vector equation of the line and the Cartesian equation of the plane is known then this can be found by:
• STEP 1: Set the position vector of the point you are looking for to have the individual components x, y, and z and substitute into the vector equation of the line
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• STEP 2: Find the parametric equations in terms of x, y, and z
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• STEP 3: Substitute these parametric equations into the Cartesian equation of the plane and solve to find λ
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• STEP 4: Substitute this value of λ back into the vector equation of the line and use it to find the position vector of the point of intersection
• STEP 5: Check this value in the Cartesian equation of the plane to make sure you have the correct answer

How do I find the point of intersection of a line and a plane in vector form?

• Suppose you have a line with equation  and plane with equation 
• Form three equations with unknowns t, λ and μ
• Solve them simultaneously on your calculator
• Substitute the values back in to get the intersection

How do I find the angle between a line and a plane?

• When you find the angle between a line and a plane you will be finding the angle between the line itself and the line on the plane that creates the smallest angle with it
  • This means the line on the plane directly under the line as it joins the plane
• It is easiest to think of these two lines making a right-triangle with the normal vector to the plane
  • The line joining the plane will be the hypotenuse
  • The line on the plane will be adjacent to the angle
  • The normal will be opposite to the angle
• As you do not know the angle of the line on the plane you can instead find the angle between the normal and the hypotenuse
  • This is the angle opposite the angle you want to find
  • This angle can be found because you will know the direction vector of the line joining the plane and the normal vector to the plane
  • This angle is also equal to the angle made by the line at the point it joins the plane and the normal vector at this point
• The smallest angle between the line and the plane will be 90° minus the angle between the normal vector and the line
  • In radians this will be  minus the angle between the normal vector and the line