Answer :

Given:- Quadrilateral ABCD with AC and BD are diagonals. P and Q are mid-point of AC and BD respectively

To Prove:- AB2 + BC2 + CD2 + DA2 = AC2 + BD2 + 4PQ2

Proof:- Let, O at Origin

be position vector of A, B, C and D respectively

As P and Q are mid-point of AC and BD,

Then, position vector of P, mid-point of AC i.e divides AC in 1:1

and position vector of Q, mid-point of BD i.e divides BD in 1:1

Section formula of internal division: Theorem given below

Let A and B be two points with position vectors

respectively, and c be a point dividing AB internally in the ration m:n. Then the position vector of c is given by


Position vector of P is given by

Position vector of Q is given by

Distance/length of PQ

Distance/length of AC

Distance/length of BD

Distance/length of AB

Distance/length of BC

Distance/length of CD

Distance/length of DA

Now, by LHS

= AB2 + BC2 + CD2 + DA2

Where are angle between vectors

Take RHS

AC2 + BD2 + 4PQ2

Thus LHS = RHS

Hence proved

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