CBSE Class 12 Maths Notes Chapter 11 Three Dimensional Geometry
Direction Cosines of a Line: If the directed line OP makes angles α, β, and γ with positive X-axis, Y-axis and Z-axis respectively, then cos α, cos β, and cos γ, are called direction cosines of a line. They are denoted by l, m, and n. Therefore, l = cos α, m = cos β and n = cos γ. Also, sum of squares of direction cosines of a line is always 1,
i.e. l2 + m2 + n2 = 1 or cos2 α + cos2 β + cos2 γ = 1
Note: Direction cosines of a directed line are unique.
Direction Ratios of a Line: Number proportional to the direction cosines of a line, are called direction ratios of a line.
(i) If a, b and c are direction ratios of a line, then =
(ii) If a, b and care direction ratios of a line, then its direction cosines are
(iii) Direction ratios of a line PQ passing through the points P(x1, y1, z1) and Q(x2, y2, z2) are x2 – x1, y2 – y1 and z2 – z1 and direction cosines are
Note:
(i) Direction ratios of two parallel lines are proportional.
(ii) Direction ratios of a line are not unique.
Straight line: A straight line is a curve, such that all the points on the line segment joining any two points of it lies on it.
Equation of a Line through a Given Point and parallel to a given vector
Vector form
where,
Cartesian form
where, (x1, y1, z1) is the point through which the line is passing through and a, b, c are the direction ratios of the line.
If l, m, and n are the direction cosines of the line, then the equation of the line is
Remember point: Before we use the DR’s of a line, first we have to ensure that coefficients of x, y and z are unity with a positive sign.
Equation of Line Passing through Two Given Points
Vector form:
Cartesian form
where, (x1, y1, z1) and (x2, y2, z2) are the points through which the line is passing.
Angle between Two Lines
Vector form: Angle between the lines
Condition of Perpendicularity: Two lines are said to be perpendicular, when in vector form
or l1l2 + m1m2 + n1n2 = 0 [direction cosine form]
Condition that Two Lines are Parallel: Two lines are parallel, when in vector form
or
[direction cosine form]
Shortest Distance between Two Lines: Two non-parallel and non-intersecting straight lines, are called skew lines.
For skew lines, the line of the shortest distance will be perpendicular to both the lines.
Vector form: If the lines are
where
Cartesian form: If the lines are
Then, shortest distance,
Distance between two Parallel Lines: If two lines l1 and l2 are parallel, then they are coplanar. Let the lines be
Note: If two lines are parallel, then they both have same DR’s.
Distance between Two Points: The distance between two points P (x1, y1, z1) and Q (x2, y2, z2) is given by
Mid-point of a Line: The mid-point of a line joining points A (x1, y1, z1) and B (x2, y2, z2) is given by
Plane: A plane is a surface such that a line segment joining any two points of it lies wholly on it. A straight line which is perpendicular to every line lying on a plane is called a normal to the plane.
Equations of a Plane in Normal form
Vector form: The equation of plane in normal form is given by
Cartesian form: The equation of the plane is given by ax + by + cz = d, where a, b and c are the direction ratios of plane and d is the distance of the plane from origin.
Another equation of the plane is lx + my + nz = p, where l, m, and n are direction cosines of the perpendicular from origin and p is a distance of a plane from origin.
Note: If d is the distance from the origin and l, m and n are the direction cosines of the normal to the plane through the origin, then the foot of the perpendicular is (ld, md, nd).
Equation of a Plane Perpendicular to a given Vector and Passing Through a given Point
Vector form: Let a plane passes through a point A with position vector
This is the vector equation of the plane.
Cartesian form: Equation of plane passing through point (x1, y1, z1) is given by
a (x – x1) + b (y – y1) + c (z – z1) = 0 where, a, b and c are the direction ratios of normal to the plane.
Equation of Plane Passing through Three Non-collinear Points
Vector form: If
Cartesian form: If (x1, y1, z1) (x2, y2, z2) and (x3, y3, z3) are three non-collinear points, then equation of the plane is
If above points are collinear, then
Equation of Plane in Intercept Form: If a, b and c are x-intercept, y-intercept and z-intercept, respectively made by the plane on the coordinate axes, then equation of plane is
Equation of Plane Passing through the Line of Intersection of two given Planes
Vector form: If equation of the planes are
where, λ is a constant and calculated from given condition.
Cartesian form: If the equation of planes are a1x + b1y + c1z = d1 and a2x + b2y + c2z = d2, then equation of any plane passing through the intersection of planes is a1x + b1y + c1z – d1 + λ (a2x + b2y + c2z – d2) = 0
where, λ is a constant and calculated from given condition.
Coplanarity of Two Lines
Vector form: If two lines
Angle between Two Planes: Let θ be the angle between two planes.
Vector form: If
Note: The planes are perpendicular to each other, if
Cartesian form: If the two planes are a1x + b1y + c1z = d1 and a2x + b2y + c2z = d2, then
Note: Planes are perpendicular to each other, if a1a2 + b1b2 + c1c2 = 0 and planes are parallel, if
Distance of a Point from a Plane
Vector form: The distance of a point whose position vector is
Note:
(i) If the equation of the plane is in the form
(ii) The length of the perpendicular from origin O to the plane
Cartesian form: The distance of the point (x1, y1, z1) from the plane Ax + By + Cz = D is
Angle between a Line and a Plane
Vector form: If the equation of line is
and so the angle Φ between the line and the plane is given by 90° – θ,
i.e. sin(90° – θ) = cos θ
Cartesian form: If a, b and c are the DR’s of line and lx + my + nz + d = 0 be the equation of plane, then
If a line is parallel to the plane, then al + bm + cn = 0 and if line is perpendicular to the plane, then
Remember Points
(i) If a line is parallel to the plane, then normal to the plane is perpendicular to the line. i.e. a1a2 + b1b2 + c1c2 = 0
(ii) If a line is perpendicular to the plane, then DR’s of line are proportional to the normal of the plane.
i.e.
where, a1, b1 and c1 are the DR’s of a line and a2, b2 and c2 are the DR’s of normal to the plane.