Q.The position vector of the point which divides the join of points 2a−3b and a+b in the ratio 3 : 1 is
(A) 23a−2b
(B) 47a−8b
(C) 43a
(D) 45a
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Section Formula
Section Formula (Vector Form)
Given two points, where is the point that divides the segment joining them in a chosen ratio? The section formula answers this with position vectors, generalising the midpoint to any ratio.
Setup
Let P and Q have position vectors a and b (measured from the origin O). We want the position vector r of the point R that divides PQ in the ratio m:n, i.e. PR:RQ=m:n.
Internal division
When R lies between P and Q:
r=m+nmb+na
Notice the cross-pairing: the far endpoint Q (position b) is weighted by m, and the near endpoint P (position a) by n. The result is a weighted average of the endpoints, so R sits closer to whichever endpoint carries the larger opposite weight.
Midpoint as a special case
Put m=n (ratio 1:1):
r=2a+b,
the familiar midpoint formula. So the section formula is just a generalised midpoint.
External division
When R lies on the line PQ but outside the segment (say beyond Q), the denominator changes sign:
r=m−nmb−na
For external division the denominator is m−n. If m=n it becomes zero — there is no finite point dividing a segment externally in an equal ratio (the point runs off to infinity).
Why it matters …
Concept: Section Formula — For vectors, if a point P divides the join of A(p) and B(q) in the ratio m:n internally, then
P=m+nnp+mq.
Step 1: Identify the given vectors.
Let A=2a−3b and B=a+b.
The ratio is 3:1, so m=3, n=1 (from A to B).
Step 2: Apply the section formula:
P=3+11⋅(2a−3b)+3⋅(a+b). …
Using the section formula for vectors, the point dividing the join of 2a−3b and a+b in the ratio 3:1 is 45a, which corresponds to option (D).
The section formula for vectors is the direct analogue of the coordinate geometry formula. If a point P divides the line segment joining A (with position vector A) and B (with position vector B) in the ratio m:n, then the position vector of P is given by:
P=m+nnA+mB
This works because we are taking a weighted average of the endpoints, where the weight of each endpoint is proportional to the distance to the opposite division point. For internal division, the formula is symmetric and intuitive: the point closer to B gets a larger weight from B, and vice versa.
Now, let’s apply this to the given problem.
-
Identify the endpoints and the ratio.
Let A=2a−3b and B=a+b.
The ratio is 3:1, meaning m=3 and n=1 (the point is closer to B since m>n).
-
Plug into the section formula.
P=m+nnA+mB=3+11⋅(2a−3b)+3⋅(a+b)
- Simplify the numerator. …
Method: Internal Division by the Section Formula (Vector Form)
Use this to find the position vector of a point dividing a segment in a given ratio.
Steps
Step 1: Identify endpoints and match the ratio to weights
For a point dividing the join of A and B internally in the ratio m:n,
r=m+nnA+mB.
Note the cross-pairing: the far endpoint B carries weight m, the near endpoint A carries weight n.
Step 2: Substitute and simplify …
Common Mistakes
Mistake 1: Swapping the weights m and n
Why it's wrong: putting m on the near endpoint gives 43(2a−3b)+1(a+b)=47a−8b — the distractor option B. Correct approach: the far endpoint B takes weight m, the near endpoint A takes weight n: r=m+nnA+mB.
Mistake 2: Using the external-division formula
Why it's wrong: the denominator m−n is for external division; this point divides internally. Correct approach: use m+n in the denominator. …
Showing the 12 most recent of 62 on this concept.
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.If iˉ+2jˉ+kˉ, αiˉ+3jˉ+2kˉ, −iˉ+4jˉ+βkˉ are the position vectors of three points A, B, C, then the position vector of a point which divides BC in the ratio α+1:β is (A) (4−1,413,49) (B) (3−1,313,39) (C) (25,27,26) (D) (37,32,31)
›Reveal solutionSolution
With A, B, C collinear, matching direction vectors pins down α and β, after which the section-formula point on BC is computed directly. The answer is (A).
Concept and Intuition
For a division ratio expressed using unknown parameters α,β to yield one specific numeric point (as the answer choices demand), those parameters must be fixed by a geometric condition on A, B, C — here, that they are collinear (a standard setup for this style of vector problem). Once α,β are pinned down, the section formula m+nnB+mC for the point dividing BC in ratio m:n finishes the problem.
Step-by-Step Solution
- AB=B−A=(α−1)iˉ+(3−2)jˉ+(2−1)kˉ=(α−1)iˉ+jˉ+kˉ.
- AC=C−A=(−1−1)iˉ+(4−2)jˉ+(β−1)kˉ=−2iˉ+2jˉ+(β−1)kˉ.
- Collinearity requires AB=tAC for some scalar t. Matching the jˉ components: 1=2t⇒t=21.
- Matching iˉ: α−1=−2t=−1⇒α=0.
- Matching kˉ: 1=(β−1)t=2β−1⇒β−1=2⇒β=3.
- So the required ratio is α+1:β=1:3.
- With α=0: B=(0,3,2); with β=3: C=(−1,4,3). …
- AP EAPCET 2026Set eng-2026-05-14-AN1 markMCQQ.Let OA=iˉ+2jˉ−4kˉ and OB=3iˉ−4jˉ−2kˉ be the position vectors of two points A and B. If a point C divides the line segment AB in the ratio 1:3 externally, then the position vector of a point which divides OC in the ratio 4:1 internally is (A) 5(iˉ−jˉ) (B) iˉ−4jˉ+2kˉ (C) 4iˉ−2jˉ+kˉ (D) 4(jˉ−kˉ)
›Reveal solutionSolution
Apply the external section formula to locate C on line AB, then apply the internal section formula on segment OC. Answer: 4(jˉ−kˉ).
Concept and Intuition
For points with position vectors A,B, the point dividing AB internally in ratio m:n is m+nmB+nA, while the point dividing it externally in ratio m:n is m−nmB−nA — the external version effectively places the dividing point beyond one of the endpoints. Once C is found this way, dividing OC internally is just the ordinary internal-section formula applied to the segment from the origin to C.
Step-by-Step Solution
- A=OA=(1,2,−4), B=OB=(3,−4,−2).
- C divides AB externally in ratio 1:3 (m=1,n=3): C=m−nmB−nA=1−31⋅B−3⋅A=−2B−3A=23A−B.
- Compute 3A=(3,6,−12), then 3A−B=(3−3,6−(−4),−12−(−2))=(0,10,−10).
- So C=2(0,10,−10)=(0,5,−5). …
- AP EAPCET 2024Set eng-2024-05-21-AN1 markMCQQ.If aˉ,bˉ,cˉ,dˉ are position vectors of 4 points such that 2aˉ+3bˉ+5cˉ−10dˉ=0ˉ, then the ratio in which the line joining cˉ and dˉ divides the line segment joining aˉ and bˉ is (A) 2:3 (B) −1:2 (C) 2:1 (D) 3:2
›Reveal solutionSolution
The given vector equation can be rearranged into a form that expresses one point as a weighted combination of the others, revealing the ratio in which the line joining cˉ and dˉ divides the segment joining aˉ and bˉ. The ratio is 3:2, so the correct option is (D).
We start with the vector equation:
2aˉ+3bˉ+5cˉ−10dˉ=0ˉ
Concept and Intuition
The Section Formula in vectors says: If a point P divides the line segment joining A and B in the ratio m:n (internally or externally), then its position vector is m+nmbˉ+naˉ (if P is between A and B, both m,n>0; if external, one is negative).
Here, we want the ratio in which the line joining cˉ and dˉ divides the segment joining aˉ and bˉ. That means: there is some point P on line AB that also lies on line CD. We need to find the ratio AP:PB (or AP:PB with sign).
The trick: Rearrange the given equation so that aˉ and bˉ appear on one side, and cˉ and dˉ on the other, then compare with the section formula.
Step-by-step solution
- Rearrange the equation to isolate terms involving aˉ and bˉ on one side:
2aˉ+3bˉ=10dˉ−5cˉ
- Factor the right-hand side to express it as a combination of cˉ and dˉ:
2aˉ+3bˉ=5(2dˉ−cˉ)
But we want a form like m+nmbˉ+naˉ for the left side, and something like p+qpdˉ+qcˉ for the right side, because the point where the lines intersect must satisfy both.
- Divide both sides by the sum of coefficients on the left (which is 2+3=5):
52aˉ+3bˉ=510dˉ−5cˉ
Simplify the right side:
52aˉ+3bˉ=2dˉ−cˉ
- Interpret the left side using the section formula: 52aˉ+3bˉ is the position vector of a point P that divides AB in the ratio 3:2 (since the coefficient of bˉ is 3 and of aˉ is 2, and the denominator is the sum). Specifically, P=3+23bˉ+2aˉ, so AP:PB=3:2 (with A at aˉ, B at bˉ). …
- AP EAPCET 2021Set eng-2021-10-05-FN1 markMCQQ.If 2a+3b−5c=0, then the ratio in which c divides AB is (A) 3 : 2 internally (B) 3 : 2 externally (C) 2 : 3 internally (D) 2 : 3 externally
›Reveal solutionSolution
Rearranging the given vector equation into the section-formula shape shows C divides AB internally in the ratio 3:2.
Concept and Intuition
The section formula says the point dividing AB internally in ratio m:n (from A to B) has position vector m+nna+mb. So whenever a vector equation can be rearranged into that exact shape, the ratio can be read off directly from the coefficients.
Step-by-Step Solution
- 2a+3b−5c=0⇒5c=2a+3b⇒c=52a+3b.
- Compare with the section formula for a point dividing AB internally in ratio m:n: m+nna+mb.
- Here n=2 (coefficient of a) and m=3 (coefficient of b), with m+n=5 matching the denominator. …
- AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQQ.The position vectors of A and B are (i^+j^+k^) and (31j^+31k^). If 'B' divides the line AC in the ratio 2:1, then position vector of 'C' is (A) (21,0,0) (B) (0,31,0) (C) (2−1,2−1,0) (D) (2−1,0,0)
›Reveal solutionSolution
Using the section formula with B dividing AC in ratio 2:1, we solve for C and get (−21,0,0).
Concept and Intuition
"B divides AC in ratio 2:1" means AB:BC=2:1, so B is closer to C. The section formula for a point dividing a segment in ratio m:n (from the first point to the second) is P=m+nn⋅(first)+m⋅(second). Here we invert this to solve for the unknown endpoint C.
Step-by-Step Solution
- B divides AC in ratio 2:1 (AB:BC=2:1), so B=2+11⋅A+2⋅C=3A+2C.
- Rearranged: 3B=A+2C⇒C=23B−A. …
- AP EAPCET 2023Set eng-2023-05-16-FN1 markMCQQ.If a point C divides the line segment joining the points with the position vectors 2iˉ−3jˉ+2kˉ and 3iˉ−jˉ−2kˉ in the ratio 2:3, then the distance of C from the point with position vector 2iˉ−jˉ+kˉ is (A) 57 (B) 54 (C) 51 (D) 53
›Reveal solutionSolution
The section formula gives C=(12/5,−11/5,2/5); the distance from C to (2,−1,1) works out to exactly 7/5.
Concept and Intuition
A point C dividing segment AB internally in ratio m:n (i.e. AC:CB=m:n) has position vector C=m+nnA+mB — the "near" endpoint gets the larger weight. Once C's coordinates are found, distance to another given point is just the standard 3D distance formula.
Step-by-Step Solution
- A=(2,−3,2), B=(3,−1,−2), ratio AC:CB=2:3, so m=2,n=3: C=2+33A+2B=53A+2B.
- 3A=(6,−9,6); 2B=(6,−2,−4); sum =(12,−11,2); divide by 5: C=(512,−511,52).
- Target point P=(2,−1,1)=(510,−55,55) (converting to fifths for easy subtraction).
- C−P=(512−10, 5−11+5, 52−5)=(52,−56,−53). …
- AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQQ.If iˉ+jˉ−kˉ, 7iˉ−2jˉ−3kˉ and −5iˉ−2jˉ+5kˉ are the position vectors of the points A,B,C respectively, then the position vector of the point of intersection of the bisector of ∠BAC and side BC is (A) 161(27iˉ−32jˉ+2kˉ) (B) 41(7iˉ−8jˉ+2kˉ) (C) 41(7iˉ+8jˉ+2kˉ) (D) 161(28iˉ−32jˉ+2kˉ)
›Reveal solutionSolution
The angle bisector from a vertex divides the opposite side in the ratio of the two adjacent sides (BD:DC=AB:AC); applying the section formula gives D=41(7iˉ−8jˉ+2kˉ).
Concept and Intuition
The internal angle-bisector theorem says the bisector of ∠A meets side BC at a point D such that BD:DC=AB:AC. Once that ratio is known, the section formula (weighted average of the endpoints, weighted inversely to the adjacent segment) locates D exactly.
Step-by-Step Solution
- A=iˉ+jˉ−kˉ, B=7iˉ−2jˉ−3kˉ, C=−5iˉ−2jˉ+5kˉ.
- AB=B−A=(6,−3,−2), so AB=36+9+4=49=7.
- AC=C−A=(−6,−3,6), so AC=36+9+36=81=9.
- By the angle-bisector theorem, BD:DC=AB:AC=7:9.
- By the section formula for internal division in ratio m:n=7:9: D=m+nn⋅B+m⋅C=169B+7C. …
- AP EAPCET 2024Set eng-2024-05-22-AN1 markMCQQ.If A=(1,2,3), B=(3,4,7) and C=(−3,−2,−5) are three points then the ratio in which the point C divides AB externally is (A) 2:3 (B) 3:2 (C) 4:3 (D) 3:4
›Reveal solutionSolution
Tests external division of a segment in 3D using the section formula; the ratio is 2:3.
Concept and Intuition
If C divides AB externally in ratio m:n, then C=m−nmB−nA. Since C, A, B are given, we can find m:n from any one coordinate and confirm with the rest — a genuine external division must satisfy ALL three coordinates simultaneously.
Step-by-Step Solution
- Let C=m−nmB−nA. Using x-coordinates: m−n3m−n=−3⇒3m−n=−3m+3n⇒6m=4n⇒nm=32.
- Check with y: m=2,n=3⇒2−34(2)−2(3)=−18−6=−2 ✓ (matches Cy=−2). …
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.On a line with direction cosines l,m,n, A(x1,y1,z1) is a fixed point. If B=(x1+4kl,y1+4km,z1+4kn) and C=(x1+kl,y1+km,z1+kn) (k>0) then the ratio in which the point B divides the line segment joining A and C is (A) 1 : 2 (B) 1 : -4 (C) 4 : -3 (D) 4 : 3
›Reveal solutionSolution
Since A,B,C are collinear points on a line with common direction cosines, the ratio in which one divides the segment joining the other two can be read off directly from the scalar multiples of the direction vector — here B lies beyond C (as seen from A), so it divides AC externally, in ratio 4:−3.
Concept and Intuition
All three points lie on the same line through direction (l,m,n), so we can parametrize every point on the line by a single scalar t via A+tk(l,m,n). Comparing the multiples of k(l,m,n) used for each point tells us instantly how far along the line each one sits, and hence the ratio of division — no need for full 3D distance/section-formula algebra.
Step-by-Step Solution
- Write each point as A+t⋅k(l,m,n):
- A corresponds to t=0
- C=A+k(l,m,n) corresponds to t=1
- B=A+4k(l,m,n) corresponds to t=4
- We want the ratio in which B divides the segment joining A and C. If B divides AC in ratio m:n (using the section formula B=m+nmC+nA), then in terms of the parameter t:
m+nm=tC−tAtB−tA is not quite it — instead use: m(tC−tB)=n(tB−tA)
More directly: nm=tC−tBtB−tA=1−44−0=−34. …
- AP EAPCET 2023Set eng-2023-05-19-FN1 markMCQQ.The coordinates of the point which divides the line joining the points (2,3,4) and (3,−4,7) in the ratio 2:4 externally is (A) (10,1,1) (B) (1,10,1) (C) (10,−10,10) (D) (1,1,10)
›Reveal solutionSolution
Apply the external section formula with A(2,3,4), B(3,−4,7), ratio 2:4. Answer: (B).
Concept and Intuition
External division extends the line segment AB beyond one of its endpoints; the formula P=m−nmB−nA (note the minus sign in the denominator, unlike internal division's m+n) locates that point.
Step-by-Step Solution
- A=(2,3,4), B=(3,−4,7), ratio m:n=2:4.
- External section formula: P=m−nmB−nA=2−42(3,−4,7)−4(2,3,4).
- Numerator: 2(3,−4,7)=(6,−8,14); 4(2,3,4)=(8,12,16); difference =(6−8, −8−12, 14−16)=(−2,−20,−2). …
- AP EAPCET 2021Set eng-2021-08-24-AN1 markMCQQ.R divides the line joining two points P and Q whose position vectors are i^+2j^−k^ and −i^+j^+k^ respectively in the ratio 2:1 externally. S divides PQ internally in the ratio 2:1. Then the position vector of the midpoint of the line joining R and S is ________ (A) 3−5i^−32j^−35k^ (B) 3−5i^+32j^+35k^ (C) 35i^−32j^−35k^ (D) 35i^+32j^+35k^
›Reveal solutionSolution
Applying the external and internal section formulas to find R and S, then averaging them, gives −35i^+32j^+35k^.
Concept and Intuition
The section formula for a point dividing PQ in ratio m:n is m+nmQ+nP internally and m−nmQ−nP externally. Applying both with P=i^+2j^−k^, Q=−i^+j^+k^ gives the two required points.
Step-by-Step Solution
- External division 2:1: R=2−12Q−P=2Q−P.
- 2Q=−2i^+2j^+2k^; R=(−2−1)i^+(2−2)j^+(2+1)k^=−3i^+0j^+3k^.
- Internal division 2:1: S=32Q+P.
- 2Q+P=(−2+1)i^+(2+2)j^+(2−1)k^=−i^+4j^+k^, so S=−31i^+34j^+31k^.
- Midpoint of R and S: M=2R+S. …
- AP EAPCET 2025Set eng-2025-05-26-FN1 markMCQQ.Let iˉ−2jˉ+kˉ, iˉ+jˉ−2kˉ, 2iˉ−jˉ−kˉ and iˉ+jˉ+kˉ be the position vectors of four points A, B, C and D respectively. If a point P divides AB in the ratio 2:1 internally and a point Q divides CD in the ratio 1:2 externally, then the ratio in which the point with position vector 5iˉ−6jˉ−5kˉ divides PQ is (A) 2:1 (B) −2:1 (C) 2:3 (D) −2:3
›Reveal solutionSolution
Compute P (internal section of AB) and Q (external section of CD) explicitly, then find in what ratio the given point divides PQ. Answer: −2:1.
Concept and Intuition
Section-formula problems are pure coordinate bookkeeping: internal division uses m+nnA+mB for ratio m:n; external division flips a sign, m−nmB−nA (equivalently substitute n→−n in the internal formula). Once P,Q are known points, finding the ratio a third point divides PQ in is a linear solve.
Step-by-Step Solution
- A=(1,−2,1), B=(1,1,−2), C=(2,−1,−1), D=(1,1,1).
- P divides AB in ratio 2:1 internally: P=2+11⋅A+2⋅B=3(1,−2,1)+(2,2,−4)=3(3,0,−3)=(1,0,−1).
- Q divides CD in ratio 1:2 externally: using the external form Q=m−nmD−nC with m=1,n=2: Q=−1D−2C=2C−D=(4,−2,−2)−(1,1,1)=(3,−3,−3).
- Let the point R=(5,−6,−5) divide PQ in ratio m:n (i.e. R=m+nnP+mQ). Using the y-coordinate (since Py=0): −6=m+n−3m⇒−6(m+n)=−3m⇒−6n=3m⇒m=−2n. …
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