Geometric Bites

Area of a Triangle in R3 in Terms of Vectors This post presents a complete derivation of the area of triangle ABC in R3 using vectors alone. The result follows directly from the definition of the cross product and basic algebraic properties. No geometric shortcuts or previously established vector identities are assumed. Every step is obtained from first principles. 1. Vector Representation of the Triangle Let A, B and C be points in R3 with position vectors A, B and C. Two sides of triangle ABC are represented by: B - A C - A If theta is the angle between these vectors, the classical formula for the area of the triangle is: Area = (1/2)*|B - A|*|C - A|*sin(theta) 2. Magnitude of the Cross Product For any vectors U and V in R3, the magnitude of the cross product is: |U x V| = |U|*|V|*sin(theta) Applying this to U = B - A and V = C - A gives: | (B - A) x (C - A) | = |B - A|*|C - A|*sin(theta) Substituting into the area formula yields: Area(ABC) = (1/2)*| (B ...

Full Coordinate Derivation of (B - A) x (C - A) in R3 This derivation shows every algebraic step involved in expanding the cross product (B - A) x (C - A) using only coordinates. No vector identities are assumed in advance. All identities that appear at the end arise directly from the coordinate formula and elementary algebra. This method provides complete transparency and is the foundation for many geometric and analytic results involving the cross product. 1. Vectors and Cross Product Formula A = (a1, a2, a3) B = (b1, b2, b3) C = (c1, c2, c3) For vectors U = (u1, u2, u3) and V = (v1, v2, v3), the cross product is defined in coordinates by: U x V = ( u2*v3 - u3*v2, u3*v1 - u1*v3, u1*v2 - u2*v1 ) This is the only formula used. Every identity later in the derivation follows from substituting coordinates into this definition. 2. Basic Cross Products Needed for the Expansion A x A A x A = ( a2*a3 - a3*a2, a3*a1 - a1*a3, a1*a2 - a2*a1 ) = (0, 0, 0) A vector...