If two vectors are represented in magnitude and direction by the two sides of a triangle taken in order, the resultant is represented by the third side of the triangle taken in the opposite order. This is called the triangle law of vectors.
In the triangle pqr shown in diagram 1 above, the side pq represents vector A, and the side qr represents vector B.
As per, the Triangle Law of Vector Addition, the vector sum of A and B will be represented by the 3rd side of the triangle in the opposite order.
That means pr will represent A + B.
Thus the triangle law of vector summation helps us to get the vector sum.