Calculateur de parallélogramme par points

The Parallelogram with Points Calculator takes the coordinates of three vertices of a parallelogram and derives the fourth, then computes area, perimeter, and diagonals. The "3 vertices determine the 4th" trick works because the diagonals of a parallelogram bisect each other — knowing three corners pins down the fourth uniquely. This tutorial covers the derivation, the cross-product area formula, and worked examples.

Why 3 points determine the 4th

Take parallelogram ABCD with three vertices known: A, B, C. The fourth vertex D must satisfy:

• AD is parallel to BC (parallelogram property)
• AD has the same length as BC

This means D is reached from A by translating along the same vector as B→C. Vector arithmetic:

D = A + (C − B)

Equivalently, using coordinates: x_D = x_A + (x_C − x_B), y_D = y_A + (y_C − y_B).

The cross-product area formula

For vectors AB = B − A and AD = D − A (the two sides from vertex A), the parallelogram area is:

Area = |AB × AD|

where × is the 2D cross product (more precisely the z-component of the 3D cross product, since we're in the plane).

For vectors u = (u_x, u_y) and v = (v_x, v_y):

|u × v| = |u_x · v_y − u_y · v_x|

Worked example 1 — find 4th vertex + area

Given A = (0, 0), B = (4, 0), C = (5, 3). Find D and compute area.

4th vertex: D = A + (C − B) = (0, 0) + (1, 3) = (1, 3).

So D = (1, 3).

Area: vectors AB = (4, 0), AD = (1, 3). Cross product = |4 × 3 − 0 × 1| = 12.

Area = 12 square units.

Verification: this parallelogram has base 4 (side AB along x-axis) and height 3 (perpendicular distance from D to AB). Area = 4 × 3 = 12. ✓

Worked example 2 — parallelogram in oblique position

A = (1, 1), B = (5, 2), C = (7, 6). Find D.

D = A + (C − B) = (1, 1) + (2, 4) = (3, 5).

Vectors AB = (4, 1), AD = (2, 4). Cross product = |4 × 4 − 1 × 2| = |16 − 2| = 14.

Area = 14.

Computing the diagonals

From the four vertices, the two diagonals are:

• Diagonal 1: from A to C — length |AC|
• Diagonal 2: from B to D — length |BD|

Each length computed via the distance formula.

The parallelogram law check

For any parallelogram with sides a, b and diagonals p, q: 2(a² + b²) = p² + q². You can verify this from the coordinate calculation as a sanity check.

Why does order of input matter?

The calculator expects three vertices in CONSECUTIVE order — A, B, C with B between A and C in the parallelogram's boundary traversal. Different orderings produce different parallelograms (or no valid parallelogram).

If the three points form an "interior" set (one of which is opposite-vertex), the calculation may produce a different shape than expected.

Real-world applications

• CAD design: defining parallelogram shapes by three corner positions, with the fourth auto-computed.
• Surveying: when only three corners are accessible (the fourth is across a river/building), compute it from the parallelogram property.
• Vector graphics: rendering parallelograms in computer graphics from three control points.
• Physics: computing parallelogram-of-forces using two force vectors' tips as B and D, origin as A.

Common mistakes

• Out-of-order vertices. Listing A, B, C in non-consecutive boundary order gives the wrong fourth vertex.
• Forgetting absolute value in cross product. The cross product can be negative (depending on vertex orientation); area is always positive — take |result|.
• Treating the parallelogram as a rectangle. A general parallelogram with three given points might not be a rectangle. Don't assume right angles.