MHT-CET Maths · Vectors
Vector Geometry — Section Formula, Triangle, and Parallelogram
Using the section formula (internal and external), the centroid and median identities, the triangle-centre formulas (incentre, orthocentre), and parallelogram relations to locate points and ratios from position vectors.
Why this matters
This is the bread-and-butter geometry strand of MHT-CET Vectors — about a dozen PYQs across the years, roughly half of them HARD. The two engines are the section formula (a point dividing a segment in a given ratio) and the centroid as the average of vertex position vectors; from those, medians, cevian-intersection ratios, the incentre, and parallelogram classification all follow. Get the internal-vs-external sign right and decide which point is weighted m versus n, and most of these resolve to a few lines of vector algebra.
Concept 1 of 5
Section formula — internal, external, and midpoint
Intuition
Definition
Let and have position vectors and , and let divide in the ratio .
- Internal division ( between and ): .
- External division ( on the line, outside ): .
- Midpoint (): .
In the internal form, the far endpoint carries the weight and the near endpoint carries , where . Memorise the form rather than a side-story.
Section formula (internal / external / midpoint)
- position vectors of the endpoints
- ratio in which divides the segment
- position vector of the dividing point
Diagram · section formula (internal vs external), m : n = 2 : 1
Internal: P = (m·b + n·a)/(m + n) sits between A and B. External: Q = (m·b − n·a)/(m − n) sits beyond B — the minus sign is what pushes it outside. The midpoint is the m = n case, (a + b)/2.
Worked example
- Internal division: with , .
- .
- .
Practice this concept4 quick reps
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Midpoint of and ?
- 2.Internal-division formula for ratio ?
- 3.External-division formula for ratio ?
- 4.Point dividing , internally ?
Internal adds, external subtracts
Which point gets the weight ?
Concept 2 of 5
Centroid and median identities
Intuition
Definition
For triangle with vertices :
- Centroid: .
- **Median through ** to the midpoint of : , equivalently .
- The centroid divides each median in ratio (vertex to centroid : centroid to midpoint).
- Tetrahedron centroid (four vertices): .
Centroid and median vector
- position vectors of the vertices
- position vector of the centroid
- midpoint of ; is the median from
Diagram · closed loop & centroid
Walking the edges A→B→C→A returns you to the start, so AB + BC + CA = 0. The three medians meet at the centroid G = (a + b + c)/3, the average of the vertices' position vectors.
Worked example
- Median vector: .
- Length: .
- .
Practice this conceptself-check · 4 quick reps
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Practice — Level 1 (4 reps)
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- 1.Centroid formula for ?
- 2.Median vector through in terms of the sides?
- 3.The centroid divides each median in what ratio (from the vertex)?
- 4.Centroid of , , ?
From the bank · past-year question
[Q145 · 9th May Shift 2 · 2024]
Median length half the side it bisects
Centroid uses position vectors, not side vectors
Concept 3 of 5
Finding the ratio, collinearity, and cevian intersection
Intuition
Definition
If lies on line with , then comparing the two sides recovers . Collinearity: are collinear iff with (i.e. divides in ratio ). Cevian intersection: write the meeting point as a section point of cevian 1 AND of cevian 2, equate, and solve the resulting linear system for the two parameters.
Ratio recovery and external division
- the ratio recovered by comparing coefficients
- external-division point — used for one branch of perpendicular-cevian problems
Worked example
- Let divide internally in ratio : .
- Compare the components: .
- Check the components: ✓ — consistent, so .
Practice this conceptself-check · 4 quick reps
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Practice — Level 1 (4 reps)
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- 1.collinear with requires?
- 2.Recover the ratio: equate the ____ of the two section-formula expressions.
- 3.External division of in ratio ?
- 4.Midpoint divides a segment in ratio?
From the bank · past-year question
[Shift || · 2025]
Internal and external points use the SAME magnitude of ratio
Cevian-intersection ratio is asked along ONE cevian
Concept 4 of 5
Incentre, orthocentre, and the angle bisector
Intuition
Definition
For triangle with vertices and opposite side lengths , , :
- Incentre: — each vertex weighted by its OPPOSITE side.
- Orthocentre: the point with and (each altitude the opposite side).
- Internal angle bisector of : along — equal coefficients on the two unit vectors.
Incentre and angle bisector
- lengths of sides opposite : , etc.
- position vector of the incentre
- sum of unit vectors — the internal-bisector direction
Worked example
- Side lengths (opposite each vertex): , , .
- .
- .
Practice this conceptself-check · 4 quick reps
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Practice — Level 1 (4 reps)
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- 1.Incentre weight on vertex ?
- 2.The orthocentre is the meeting point of the triangle's ____.
- 3.Internal bisector of points along?
- 4.Centroid vs incentre: which uses side-length weights?
From the bank · past-year question
[Q147 · 2nd May Shift 2 · 2023]
Incentre weights are OPPOSITE side lengths
Bisector uses unit vectors — sum, not difference
Orthocentre centroid circumcentre
Concept 5 of 5
Triangle and parallelogram applications
Intuition
Definition
Right angle at a vertex: iff . Quadrilateral classification from vertices (in order):
- Opposite sides equal as vectors () parallelogram.
- A parallelogram with (equal diagonals) rectangle.
- A parallelogram with (perpendicular diagonals) rhombus.
- Both equal AND perpendicular diagonals square.
Right-angle test and parallelogram diagonals
- the two side-vectors leaving the right-angle vertex
- diagonals of quadrilateral
Diagram · parallelogram diagonals = a + b and a − b
From a shared corner, sides a and b span the parallelogram. The diagonal from that corner is a + b; the diagonal between the side tips is a − b. They bisect each other, and |a + b|² + |a − b|² = 2(|a|² + |b|²).
Worked example
- Side-vectors at : , .
- Right angle at : .
- .
Practice this conceptself-check · 4 quick reps
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Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Right angle at iff
- 2.Equal opposite side-vectors imply which quadrilateral?
- 3.Parallelogram with perpendicular diagonals is a?
- 4.Parallelogram with equal diagonals is a?
From the bank · past-year question
[Q102 · 15th May Shift 2 · 2023]
Equal diagonals rectangle, perpendicular diagonals rhombus
Right-angle test needs side-vectors FROM the vertex
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (5)
- Section formula — internal, external, and midpoint
Section formula (internal / external / midpoint)
- Centroid and median identities
Centroid and median vector
- Finding the ratio, collinearity, and cevian intersection
Ratio recovery and external division
- Incentre, orthocentre, and the angle bisector
Incentre and angle bisector
- Triangle and parallelogram applications
Right-angle test and parallelogram diagonals
Watch out for (11)
- Internal adds, external subtracts→ Section formula — internal, external, and midpoint
- Which point gets the weight ?→ Section formula — internal, external, and midpoint
- Median length half the side it bisects→ Centroid and median identities
- Centroid uses position vectors, not side vectors→ Centroid and median identities
- Internal and external points use the SAME magnitude of ratio→ Finding the ratio, collinearity, and cevian intersection
- Cevian-intersection ratio is asked along ONE cevian→ Finding the ratio, collinearity, and cevian intersection
- Incentre weights are OPPOSITE side lengths→ Incentre, orthocentre, and the angle bisector
- Bisector uses unit vectors — sum, not difference→ Incentre, orthocentre, and the angle bisector
- Orthocentre centroid circumcentre→ Incentre, orthocentre, and the angle bisector
- Equal diagonals rectangle, perpendicular diagonals rhombus→ Triangle and parallelogram applications
- Right-angle test needs side-vectors FROM the vertex→ Triangle and parallelogram applications
Mastery check — 5 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
[Q129 · 16th May Shift 1 · 2023]
[Q119 · 4th May Shift 1 · 2023]
[Q111 · 4th May Shift 2 · 2023]
[Q140 · 14th May Shift 1 · 2024]
[Q122 · 3rd May Shift 2 · 2023]
Drill every past-year question on this subtopic
12 questions from the bank — paginated, with cart and Word-export support.