Q.The vector equation of the line through the points (3,4,−7) and (1,−1,6) is __________.
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Vector Equation of a Line
A line is fixed by two pieces of information: one point it passes through and the direction it runs in. The vector equation packages both.
Let a be the position vector of a known point A on the line, and let b be any vector parallel to the line (its direction). For any point P on the line with position vector r, the displacement AP points along the line, so it is a scalar multiple of b: AP=λb. Since r=a+AP,
r=a+λb,λ∈R
How to read it
As the parameter λ runs through all real numbers, r traces every point of the line. At λ=0 you sit at A; positive λ moves one way along b, negative λ the other. Think of a as "where you start" and λb as "how far and which way you walk."
Line through two points
If the line passes through points with position vectors a and b, its direction is b−a, so
r=a+λ(b−a)
Example
The line through A(1,2,−1) parallel to b=2i^−j^+3k^ is
r=(i^+2j^−k^)+λ(2i^−j^+3k^). …
Concept: Vector Equation of a Line — a line through two points A and B is given by r=a+λ(b−a), where a and b are position vectors of the points.
Step 1: Let a=3i^+4j^−7k^ and b=i^−j^+6k^.
Step 2: Direction vector d=b−a=(1−3)i^+(−1−4)j^+(6+7)k^=−2i^−5j^+13k^.
Step 3: Using r=a+λd, the equation is: …
The vector equation of a line is r=a+λb, where a is a point on the line and b is the direction vector. For points (3,4,−7) and (1,−1,6), the direction vector is (1−3,−1−4,6−(−7))=(−2,−5,13). So the equation is r=(3,4,−7)+λ(−2,−5,13).
To write the vector equation of a line, you need two things: a position vector of any point on the line, and a direction vector that tells you which way the line runs. Think of it like this: you start at a fixed point (the position vector), and then you can move any distance along the direction vector — that sweep gives you every point on the line.
The general form is:
r=a+λb
where a is the position vector of a known point, b is a direction vector parallel to the line, and λ is a scalar parameter (any real number).
Now let’s apply this to the given points.
- Pick a point on the line. You can choose either of the two given points. Let’s take (3,4,−7). Its position vector is:
a=3i^+4j^−7k^
(If you chose the other point, the final equation would look different but describe the same line — that’s fine.)
- Find the direction vector. The direction vector is simply the vector from one point to the other. Subtract the coordinates of the first point from the second (or vice versa — the direction just flips sign, which is still parallel). From (3,4,−7) to (1,−1,6):
b=(1−3)i^+(−1−4)j^+(6−(−7))k^=−2i^−5j^+13k^
So b=(−2,−5,13). …
Method: Vector Equation of a Line Through Two Given Points
Use this whenever a line is fixed by two points it passes through rather than by a stated direction.
Steps
Step 1: Turn each point into a position vector.
For points A and B, write a and b as xi^+yj^+zk^.
Step 2: Build the direction from the two points.
A line's direction is the displacement from one point to the other:
d=b−a.
Subtract componentwise. Reversing the order only flips the sign, and −d runs the same line the opposite way, so either order is valid. Watch double negatives, e.g. 6−(−7)=13.
Step 3: Write the equation with one point plus the direction. …
Common Mistakes
Mistake 1: Using a point's position vector as the direction.
Why it's wrong: a single point gives no direction; the line's direction comes from the difference of the two points, b−a. Correct approach: always subtract the two position vectors to get the direction before writing the equation.
Mistake 2: Subtracting the coordinates inconsistently. …
Showing the 12 most recent of 16 on this concept.
- AP EAPCET 2022Set eng-2022-07-04-FN1 markMCQQ.The point of intersection of the lines rˉ=2bˉ+t(6cˉ−aˉ) and rˉ=aˉ+s(bˉ−3cˉ) is (A) aˉ+bˉ+cˉ (B) bˉ−cˉ−6aˉ (C) 2aˉ−bˉ+cˉ (D) aˉ+2bˉ−6cˉ
›Reveal solutionSolution
Matching coefficients of the (linearly independent) position vectors aˉ,bˉ,cˉ on both sides of the line equations pins down the parameters and gives the intersection point aˉ+2bˉ−6cˉ.
Concept and Intuition
When two vector lines are given in terms of a common set of independent reference vectors, the intersection point can be found by comparing the coefficients of each reference vector on both sides — this is valid because aˉ,bˉ,cˉ are linearly independent (as position vectors of non-collinear/non-coplanar reference points), so a vector equation between them is only satisfied if each coefficient matches independently.
Step-by-Step Solution
- Line 1: rˉ=2bˉ+t(6cˉ−aˉ)=−taˉ+2bˉ+6tcˉ.
- Line 2: rˉ=aˉ+s(bˉ−3cˉ)=aˉ+sbˉ−3scˉ.
- Equate coefficients of aˉ: −t=1⇒t=−1.
- Equate coefficients of bˉ: 2=s⇒s=2. …
- AP EAPCET 2021Set eng-2021-08-20-FN1 markMCQQ.The line passing through (1,1,−1) and parallel to the vector i^+2j^−k^ meets the line −1x−3=5y+2=−4z−2 at A and the plane 2x−y+2z+7=0 at B. Then AB= (A) 6 (B) 26 (C) 36 (D) 46
›Reveal solutionSolution
Parametrizing the line and solving for its intersections with the given line and the given plane locates A=(2,3,−2), B=(4,7,−4), giving AB=26.
Concept and Intuition
A line in space is fully described by a point and a direction vector; every point on it is found by a single parameter t. Intersecting it with another line means finding a common point (solve simultaneously for both parameters); intersecting it with a plane means substituting the parametrized coordinates into the plane equation and solving for t.
Step-by-Step Solution
- Parametrize the line through (1,1,−1) with direction (1,2,−1): (x,y,z)=(1+t,1+2t,−1−t).
- Parametrize the second line as (3−s,−2+5s,2−4s) (from −1x−3=5y+2=−4z−2=s).
- Equating: 1+t=3−s⇒t+s=2; 1+2t=−2+5s⇒2t−5s=−3; −1−t=2−4s⇒t=4s−3.
- From the first, s=2−t; substitute into the third: t=4(2−t)−3=5−4t⇒5t=5⇒t=1, so s=1 (checked consistent with the second equation).
- So A=(1+1,1+2,−1−1)=(2,3,−2). …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.Points P and Q are given by OP=iˉ−jˉ−kˉ and OQ=−iˉ+jˉ+kˉ. A line along the vector aˉ=iˉ+jˉ passes through the point P and another line along the vector bˉ=jˉ−kˉ passes through the point Q. If a line along the vector cˉ=iˉ−jˉ+kˉ intersects both the lines along the vectors aˉ and bˉ at L and M respectively, then PM= (A) iˉ−jˉ+2kˉ (B) 4iˉ+4jˉ (C) −2iˉ+10jˉ−6kˉ (D) 3iˉ−2jˉ+kˉ
›Reveal solutionSolution
Setting up the two given lines parametrically and forcing the connecting segment LM to be parallel to cˉ pins down the parameters, giving PM=−2iˉ+10jˉ−6kˉ.
Concept and Intuition
When a third line (direction cˉ) is said to intersect two given (skew) lines, the two intersection points L (on line a) and M (on line b) must satisfy that the vector between them, M−L, is itself a scalar multiple of cˉ (since both points lie on the same line of direction cˉ). This turns a geometric intersection condition into a simple vector equation to solve for the two free parameters.
Step-by-Step Solution
- P=(1,−1,−1) with OP=iˉ−jˉ−kˉ. Line 1 (through P, direction aˉ=iˉ+jˉ): L=(1+s,−1+s,−1) for parameter s.
- Q=(−1,1,1) with OQ=−iˉ+jˉ+kˉ. Line 2 (through Q, direction bˉ=jˉ−kˉ): M=(−1,1+u,1−u) for parameter u.
- Since L and M both lie on the line of direction cˉ=iˉ−jˉ+kˉ, we need M−L=kcˉ for some scalar k:
M−L=(−1−(1+s),1+u−(−1+s),1−u−(−1))=(−2−s,2+u−s,2−u)
- Equate components to k(1,−1,1): −2−s=k; 2+u−s=−k; 2−u=k.
- From the first and third: −2−s=2−u⇒u−s=4.
- From the second: 2+u−s=−(−2−s)=2+s⇒u−s=s⇒u=2s. …
- AP EAPCET 2025Set eng-2025-05-21-AN1 markMCQQ.The point of intersection of the lines represented by rˉ=(iˉ−6jˉ+2kˉ)+t(iˉ+2jˉ+kˉ) and rˉ=(4jˉ+kˉ)+s(2iˉ+jˉ+2kˉ) is (A) 8iˉ+9jˉ+10kˉ (B) 8iˉ+8jˉ+7kˉ (C) 8iˉ+9jˉ+8kˉ (D) 8iˉ+8jˉ+9kˉ
›Reveal solutionSolution
Solving the three coordinate equations for the two line parameters (and verifying consistency) locates the intersection point as (8,8,9).
Concept and Intuition
Two lines in space intersect only if there's a common point — that is, values of the two parameters t and s that make all three coordinates match simultaneously. With two unknowns and three equations, the system is over-determined; solving two of the equations for t,s and then checking the third confirms genuine intersection (as opposed to skew lines).
Step-by-Step Solution
- Line 1: rˉ=(iˉ−6jˉ+2kˉ)+t(iˉ+2jˉ+kˉ), giving coordinates (1+t, −6+2t, 2+t).
- Line 2: rˉ=(4jˉ+kˉ)+s(2iˉ+jˉ+2kˉ), giving coordinates (2s, 4+s, 1+2s).
- Equating x: 1+t=2s — (i). Equating y: −6+2t=4+s — (ii). Equating z: 2+t=1+2s — (iii).
- From (i): t=2s−1. Substitute into (ii): −6+2(2s−1)=4+s⇒−6+4s−2=4+s⇒4s−8=4+s⇒3s=12⇒s=4.
- Then t=2(4)−1=7.
- Verify with (iii): 2+t=2+7=9 and 1+2s=1+8=9 — consistent, so the lines genuinely intersect. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.iˉ−2jˉ is a point on the line parallel to the vector 2iˉ+kˉ. If iˉ+2jˉ is a point on the plane parallel to the vectors 2jˉ−kˉ and iˉ+2kˉ, then the point of intersection of the line and the plane is (A) −31(iˉ+6jˉ+2kˉ) (B) 31(iˉ+6jˉ+2kˉ) (C) −31(iˉ−6jˉ+2kˉ) (D) 31(iˉ−6jˉ+2kˉ)
›Reveal solutionSolution
Writing the line and plane in coordinates and substituting the line's parametric form into the plane's equation gives t=−2/3, landing exactly on option (A).
Concept and Intuition
A line "parallel to a vector through a point" and a "plane parallel to two vectors through a point" are both standard 3D-geometry objects: the line is a one-parameter family, the plane's normal is the cross product of its two direction vectors. Finding their intersection is just substituting the line's parametrization into the plane's Cartesian equation and solving for the parameter.
Step-by-Step Solution
- The line passes through P0=iˉ−2jˉ=(1,−2,0) and is parallel to 2iˉ+kˉ=(2,0,1). Parametrize: (x,y,z)=(1+2t,−2,t).
- The plane passes through Q0=iˉ+2jˉ=(1,2,0) and is parallel to 2jˉ−kˉ=(0,2,−1) and iˉ+2kˉ=(1,0,2).
- Normal to the plane: n=(0,2,−1)×(1,0,2). Compute: nx=2(2)−(−1)(0)=4, ny=(−1)(1)−0(2)=−1, nz=0(0)−2(1)=−2. So n=(4,−1,−2).
- Plane equation: 4(x−1)−1(y−2)−2(z−0)=0⇒4x−y−2z=2.
- Substitute the line's coordinates: 4(1+2t)−(−2)−2t=2⇒4+8t+2−2t=2⇒6+6t=2⇒t=−32.
- Point of intersection: x=1+2(−32)=1−34=−31, y=−2, z=−32. …
- AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQQ.Line L1 passes through the points iˉ+jˉ and kˉ−iˉ. Line L2 passes through the point jˉ+2kˉ and is parallel to the vector iˉ+jˉ+kˉ. If xiˉ+yjˉ+zkˉ is the point of intersection of the lines L1 and L2, then (y−x)= (A) 2z (B) −2z (C) z (D) −z
›Reveal solutionSolution
Parametrise both lines, equate coordinates to find the common point, then check the relation between y−x and z at that point.
Concept and Intuition
Two lines given by a point + direction (or two points) can be intersected by writing both as parametric equations in 3D and solving the resulting system for the two parameters — the point where they agree (if it exists) is the intersection.
Step-by-Step Solution
- L1 passes through A=(1,1,0) (iˉ+jˉ) and B=(−1,0,1) (kˉ−iˉ); direction =B−A=(−2,−1,1).
- Parametrise: L1:(x,y,z)=(1−2t, 1−t, t).
- L2 passes through (0,1,2) (jˉ+2kˉ), parallel to (1,1,1): (x,y,z)=(s, 1+s, 2+s).
- Equate: 1−2t=s, 1−t=1+s, t=2+s.
- From the 2nd equation: −t=s, i.e. s=−t. Substitute into the 3rd: t=2−t⇒2t=2⇒t=1, s=−1.
- Check the 1st equation: 1−2(1)=−1=s. Consistent. …
- AP EAPCET 2023Set eng-2023-05-19-FN1 markMCQQ.Let a,b,c be three non-coplanar vectors. Then the point of intersection of the line joining the points a+b+c, a−b+3c and the line joining the points 2a−b+c, a−2b+4c is (A) 2a+4c (B) 3a−3b+5c (C) a−2b+4c (D) a−b+3c
›Reveal solutionSolution
Parametrize both lines in terms of the (linearly independent) basis a,b,c and match coefficients to find where they meet. Answer: (C).
Concept and Intuition
Since a,b,c are non-coplanar, they act like an independent coordinate basis (like i,j,k), so two vectors expressed in this basis are equal only if all three coefficients match separately.
Step-by-Step Solution
- Line 1 through a+b+c and a−b+3c: parametrize as L1(t)=(a+b+c)+t[(a−b+3c)−(a+b+c)]=a+(1−2t)b+(1+2t)c.
- Line 2 through 2a−b+c and a−2b+4c: parametrize as L2(s)=(2a−b+c)+s[(a−2b+4c)−(2a−b+c)]=(2−s)a+(−1−s)b+(1+3s)c.
- Setting L1(t)=L2(s) and matching the a-coefficients: 1=2−s⇒s=1.
- Matching b-coefficients: 1−2t=−1−s=−2⇒t=23. …
- AP EAPCET 2024Set eng-2024-05-20-FN1 markMCQQ.Let O(0ˉ), A(iˉ+2jˉ+kˉ), B(−2iˉ+3kˉ), C(2iˉ+jˉ), D(4kˉ) are position vectors of the points O, A, B, C and D. If a line passing through A and B intersects the plane passing through O, C and D at the point R, then position vector of R is (A) −8iˉ−4jˉ+7kˉ (B) 2iˉ+jˉ+kˉ (C) −7iˉ−6jˉ−5kˉ (D) 3iˉ+2jˉ−5kˉ
›Reveal solutionSolution
Parametrize the line AB, intersect it with the plane x=2y (through O,C,D), and get R=−8iˉ−4jˉ+7kˉ.
Concept and Intuition
A line meeting a plane is found by writing the line in parametric form, substituting into the plane's Cartesian equation, and solving for the parameter. The plane through three points O,C,D (one of which is the origin) has normal OC×OD and passes through the origin, so its equation has zero constant term.
Step-by-Step Solution
- A=(1,2,1), B=(−2,0,3). Line AB: P(t)=A+t(B−A)=(1,2,1)+t(−3,−2,2)=(1−3t,2−2t,1+2t).
- C=(2,1,0), D=(0,0,4). Plane through O,C,D is spanned by OC=(2,1,0) and OD=(0,0,4); normal =OC×OD=iˉ20jˉ10kˉ04=(4,−8,0).
- Plane equation (through origin): 4x−8y=0⇒x=2y. …
- AP EAPCET 2025Set eng-2025-05-26-FN1 markMCQQ.If the line joining the points iˉ+2jˉ and jˉ−2kˉ intersects the plane passing through the points 2iˉ−jˉ, 2jˉ+3kˉ and kˉ−2iˉ at rˉ, then rˉ.(iˉ+jˉ+kˉ)= (A) 15 (B) 5 (C) 3 (D) 7
›Reveal solutionSolution
Find the plane through three given points, parametrize the given line, intersect, then dot with (1,1,1). Answer: 15.
Concept and Intuition
A line-meets-plane problem is mechanical once both objects are in coordinate form: get the plane's normal via a cross product of two side vectors, write the line parametrically, substitute, and solve for the parameter at the intersection.
Step-by-Step Solution
- Points: A=(2,−1,0), B=(0,2,3), C=(−2,0,1) (from 2iˉ−jˉ, 2jˉ+3kˉ, kˉ−2iˉ).
- AB=B−A=(−2,3,3), AC=C−A=(−4,1,1).
- Normal n=AB×AC=(3⋅1−3⋅1, −[(−2)(1)−(3)(−4)], (−2)(1)−(3)(−4))=(0,−10,10), simplify to (0,−1,1).
- Plane through A with this normal: −1(y−(−1))+1(z−0)=0⇒z−y=1. (Check B: 3−2=1 ✓; C: 1−0=1 ✓.)
- Line through P1=(1,2,0) and P2=(0,1,−2): direction d=P2−P1=(−1,−1,−2), so rˉ(t)=(1−t, 2−t, −2t). …
- AP EAPCET 2021Set eng-2021-08-20-AN1 markMCQQ.The point of intersection of the lines joining points i^+2j^,2i^−j^ and −i^,2i^ is (A) 35i^ (B) 53i^+j^ (C) 5−3i^ (D) 52j^
›Reveal solutionSolution
One of the two lines is simply the x-axis; intersecting the other line with y=0 gives the point 35i^.
Concept and Intuition
When two given points share the same y-coordinate, the line through them is just that horizontal line — a useful shortcut that avoids solving two line equations simultaneously.
Step-by-Step Solution
- Convert to Cartesian points: i^+2j^=(1,2); 2i^−j^=(2,−1); −i^=(−1,0); 2i^=(2,0).
- The second pair, (−1,0) and (2,0), both lie on y=0, so that line is exactly the x-axis.
- The first line passes through (1,2) and (2,−1): slope =2−1−1−2=−3.
- Equation: y−2=−3(x−1)⇒y=2−3x+3=5−3x.
- Set y=0 (intersection with the x-axis): 0=5−3x⇒x=35. …
- AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQQ.Let OA=iˉ+4kˉ be the position vector of a point A. If the line passing through the point A and parallel to the vector 3iˉ+jˉ and the plane passing through the points 2iˉ+jˉ, jˉ−2kˉ and 2kˉ−iˉ intersect at the point P then ∣AP∣= (A) 0 (B) 217 (C) 17 (D) 1
›Reveal solutionSolution
The line through A actually meets the plane exactly at A itself (i.e. A lies on the plane), so the intersection point P=A and ∣AP∣=0.
Concept and Intuition
To find where a line meets a plane, parametrize the line, substitute into the plane's Cartesian equation, and solve for the parameter. If the point A (parameter t=0) itself already satisfies the plane equation, the "intersection point" coincides with A, and the segment length is trivially zero.
Step-by-Step Solution
- A=(1,0,4) (from OA=iˉ+4kˉ). The line through A parallel to 3iˉ+jˉ is r(t)=(1+3t, t, 4) — the z-coordinate never changes since the direction vector has zero kˉ component.
- Plane points: P1=(2,1,0), P2=(0,1,−2), P3=(−1,0,2).
- P2−P1=(−2,0,−2), P3−P1=(−3,−1,2). Normal =(P2−P1)×(P3−P1)=(−2,10,2), simplify to (−1,5,1).
- Plane equation using P1: −1(x−2)+5(y−1)+1(z−0)=0⇒−x+5y+z=3. …
- AP EAPCET 2021Set eng-2021-08-20-FN1 markMCQQ.Let a=i^ and b=j^. The point of intersection of the lines r×a=b×a and r×b=a×b is (A) r=i^+j^ (B) r=i^−j^ (C) r=k^ (D) r=2i^+j^
›Reveal solutionSolution
Each cross-product equation forces r onto a specific line; solving both lines simultaneously gives r=i^+j^.
Concept and Intuition
An equation of the form r×c=d×c (with d fixed and c a fixed direction) rearranges to (r−d)×c=0, meaning r−d is parallel to c — i.e. r traces out the line through the tip of d in direction c. Two such conditions together pin down a unique point: the intersection of the two lines.
Step-by-Step Solution
- From r×a=b×a: (r−b)×a=0, so r−b is parallel to a=i^. Thus r=b+ti^=j^+ti^=(t,1,0).
- From r×b=a×b: (r−a)×b=0, so r−a is parallel to b=j^. Thus r=a+sj^=i^+sj^=(1,s,0).
- Equate the two parametrizations: (t,1,0)=(1,s,0)⇒t=1, s=1. …
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