# Edexcel A Level Further Maths: Core Pure:复习笔记6.2.1 Equations of planes

### 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

#### Exam Tip

• The formula for the vector equation of a plane is given in the formula booklet, make sure you know what each part means
• Be careful to use different letters, e.g. λ and μ as the scalar multiples of the two direction vectors

#### Worked Example

The points A, B and C have position vectors  respectively, relative to the origin O.

(a) Find the vector equation of the plane.

(b) Determine whether the point D with coordinates (-2, -3, 5) lies 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 the 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

#### Exam Tip

• In an exam, using whichever form of the equation of the plane to write down a normal vector to the plane is always a good starting point

#### Worked Example

A plane  has equation . Find the equation of the plane in its Cartesian form.