Equations of planes (Edexcel A Level Further Maths): Revision Note

Equation of a Plane in Vector Form

How do I find the vector equation of a plane?

• A plane is a flat surface which is two-dimensional

  • Imagine a flat piece of paper that continues on forever in both directions

• A plane in often denoted using the capital Greek letter Π

• The vector form of the equation of a plane can be found using two direction vectors on the plane

  • The direction vectors must be

    • parallel to the plane

    • not parallel to each other

    • therefore they will intersect at some point on the plane

• The formula for finding the vector equation of a plane is

  • 

    • Where r is the position vector of any point on the plane

    • a is the position vector of a known point on the plane

    • b and c are two non-parallel direction (displacement) vectors parallel to the plane

    • s and t are scalars

• The formula can also be written as

  • 

    • Where r is the position vector of any point on the plane

    • a, b, c are the position vectors of known points on the plane

    • λ and μ are scalars

  • These formulae are given in the formula booklet but you must make sure you know what each part means

• As a could be the position vector of any point on the plane and b and c could be any non-parallel direction vectors on the plane there are infinite vector equations for a single plane

How do I determine whether a point lies on a plane?

• Given the equation of a plane  then the point r with position vector is on the plane if there exists a value of λ and μ such that

  • 

  • This means that there exists a single value of λ and μ that satisfy the three parametric equations:

    • 

    • 

    • 

• Solve two of the equations first to find the values of λ and μ that satisfy the first two equation and then check that this value also satisfies the third equation

• If the values of λ and μ do not satisfy all three equations, then the point r does not lie on the plane

Equation of a Plane in Cartesian Form

How do I find the vector equation of a plane in cartesian form?

• The cartesian equation of a plane is given in the form

  • 

  • This is given in the formula booklet

• A normal vector to the plane can be used along with a known point on the plane to find the cartesian equation of the plane

  • The normal vector will be a vector that is perpendicular to the plane

• The scalar product of the normal vector and any direction vector on the plane will be zero

  • The two vectors will be perpendicular to each other

  • The direction vector from a fixed-point A to any point on the plane, R can be written as r – a

  • Then n ∙ (r – a) = 0 and it follows that (n ∙ r) – (n ∙ a) = 0

• This gives the equation of a plane using the normal vector:

  • n ∙ r = a ∙ n

    • Where r is the position vector of any point on the plane

    • a is the position vector of a known point on the plane

    • n is a vector that is normal to the plane

  • This is given in the formula booklet

• If the vector r is given in the form  and a and n are both known vectors given in the form and  then the Cartesian equation of the plane can be found using:

  • 

  • 

  • Therefore 

  • This simplifies to the form 

    • A version of this is given in the formula booklet

How do I find the equation of a plane in Cartesian form given the vector form?

• Given the equation of the plane

  • Form three equations

    • 

    • 

    • 

• Choose a pair of equations and use them to form an equation without μ

• Choose another pair and form another equation without μ

• Use your two expressions to form an equation without μ and λ

• Rewrite the equation in the form