What is Parallelogram?
A Parallelogram is a four-sided quadrilateral whose opposite sides are parallel and congruent to each other. The opposite angles of a parallelogram are equal. The parallelogram and a rectangle are near about the same with one distinguishing property that the rectangle has all the angles of 90°0 and that of parallelogram does not.
In Mathematics, the parallelogram law belongs to elementary Geometry. This law is also known as parallelogram identity. It states that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the lengths of the two diagonals. In this article, let us study the definition of a parallelogram law, proof, and parallelogram law of vectors in detail.
Parallelogram Law of Addition
Parallelogram law of addition states that the sum of the squares of the length of the four sides of a parallelogram equals the sum of the squares of the length of the two diagonals. In Euclidean geometry, it is a must that the parallelogram should have equal opposite sides.
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If ABCD is a parallelogram, then AD = BC and AB = DC. Then according to the definition of the parallelogram law, it is stated as
2(AB)2 + 2(BC)2 = (AC)2 + (BD)2.
If a parallelogram is a rectangle, then the law is stated as
2(AB)2 + 2(BC)2 = 2(AC)2
Because in a rectangle, two diagonals are of equal lengths. i.e., (AC=BD)
Parallelogram Law of Vectors
If two vectors say vector p and vector q are acting simultaneously at a point, then it can be represented both in magnitude and direction by the adjacent sides drawn from a
point. Therefore, the resultant vector is completely represented both in direction and magnitude by the diagonal of the parallelogram passing through the point.
Consider the above figure,
The vector P represents the side OA and vector Q represents the side OB, respectively.
According to the parallelogram law, the side OC diagonal of the parallelogram represents the resultant vector R.
Vector OA + Vector OB = Vector OC
→ → →
P + Q = R
Parallelogram Law of Addition of Vectors Procedure
Following are the steps for the parallelogram law of addition of vectors:
Draw a vector using a suitable scale in the direction of the vector.
Draw the second vector using the same scale from the tail of the first vector.
Treat these vectors as the adjacent sides and complete the parallelogram.
Now, the diagonal represents the resultant vector in both magnitude and direction.
Parallelogram Law Proof
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Step 1: Let AD=BC = p, AB = DC = q, and ∠ BAD = α
Step 2: Using the law of cosines in the BAD, we get
p2+ q2 – 2pqcos(α) = BD2 ——-(1)
Step 3: We know that in a parallelogram, the adjacent angles are supplementary, so it sums up 180°0. So
∠ADC = 180 – α
Step 4: Now, again use the law of cosines in the ADC
p2 + q2 – 2pqcos(180 – α) = AC2 ——-(2)
Step 5: Apply trigonometric identity cos(180 – x) = – cos x in step (2)
p2 + q2 + 2pqcos(α) = AC2
Step 6: Now, take the sum of the squares of the diagonals adding equations 1 and 2
BD2 + AC2 =p2 + q2 – 2pqcos(α) + p2 + q2 + 2pqcos(α)
BD2 + AC2 =2p2 + 2q2 ——-(3)
BD2 + AC2 = 2(AB)2 + 2( BC)2
Hence, the parallelogram law is proved.
FAQs on Parallelogram Law
1. What are vectors?
A Physical quantity that has both magnitude and direction is said to be a vector. In a line, the length of a line is a magnitude and the arrow is its direction. The start point is its tail and the endpoint is its head.
An increase and decrease in temperature is the best example of a vector, it has both magnitude and direction. There are ten types of vectors and they are as follows:
Zero vector
Unit Vector
Position Vector
Co-initial Vector
Like and Unlike Vectors
Co-planar Vector
Collinear Vector
Equal Vector
Displacement Vector
Negative of a Vector
2. What is the Triangle Laws of Vector Addition?
Suppose, we have two vectors A and B as shown.
To add these two vectors simply place the head of one vector over the tail of the other vector as shown.
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Now join the other endpoints of both the vectors together as shown,
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The result of the addition of given vectors is given by vector C which represents the sum of vectors A and B.
i.e. C = A + B
Vector addition is commutative in nature i.e. C = A + B = B + A
Similarly, if we have to subtract both the vectors using the triangle law, then we simply reverse the direction of any vector and add it to another one as shown.
Now we can mathematically represent this as
C = A – B.
3. What are the properties of a parallelogram?
Most importantly the properties of a parallelogram are defined as a special type of quadrilateral which has a figure with two pairs of opposite sides that are parallel. In addition to this, the opposite angles in a parallelogram are equal and their diagonals bisect each other.
4. Who discovered the law of parallelogram?
Stevinus (1548–1620) was the first to demonstrate that forces could be combined by representing them by arrows to some suitable scale, and then forming a parallelogram in which the diagonal represents the sum of the two forces. All vectors must combine in this manner. For example, if F1 and F2 are two forces, the resultant can be found by constructing the parallelogram.
