**Vectors can be added by parallelogram law of addition**

The parallelogram law of addition states that when two vectors are represented by two adjacent sides of a parallelogram, their vector sum is given by the diagonal of the parallelogram that starts from the same point as the two vectors.

To understand this concept, let's consider two vectors A and B.

1. **Drawing vectors A and B**
- Draw vector A as an arrow with its initial point at the origin.
- Draw vector B as an arrow with its initial point at the terminal point of vector A.
- The arrowheads of both vectors should be pointing in the same direction.

2. **Completing the parallelogram**
- Complete the parallelogram by drawing lines parallel to vectors A and B from the terminal point of vector B.
- The parallelogram should have vector A as one of its adjacent sides and vector B as the other adjacent side.

3. **Drawing the resultant vector**
- The diagonal of the parallelogram, starting from the same point as vectors A and B, represents the resultant vector.
- The resultant vector is the vector sum of vectors A and B.

4. **Finding the magnitude and direction of the resultant vector**
- To find the magnitude of the resultant vector, measure the length of the diagonal using a ruler.
- To find the direction of the resultant vector, measure the angle the diagonal makes with vector A or vector B.

5. **Adding vectors algebraically**
- Alternatively, vectors can also be added algebraically.
- The x-components and y-components of the vectors can be added separately to get the resultant vector.
- The magnitude of the resultant vector can be found using the Pythagorean theorem, and its direction can be found using trigonometry.

6. **Other methods of vector addition**
- Vector addition can also be done using the head-to-tail method or the component method.
- However, the parallelogram law of addition is particularly useful when the vectors are not collinear or when dealing with multiple vectors.

By following the parallelogram law of addition, vectors can be added accurately, taking into account both magnitude and direction. This method is widely used in physics, engineering, and other fields that involve vector calculations.

Definitely parallelogram law of vectors.
The direct logic for this is that a given physical quantity can be said to be a vector quantity only when it follows the following conditions:
1) It must have a magnitude
2) it must have a spatial direction
3) it must follow the law of triangle addition.
4) it must follow the law of parallelogram of addition
5) it must follow the associative law.
Therefore, according to point 4, for adding two vector quantities we always need to use the law of parallelogram addition.