Question

If in addition law VECTOR A + VECTOR B = RESULTANT VECTOR so why we use Pythagoras theorem in right 🔺 to found a resultant and why we do not add the vectors.. .....

Answer 1

when we say, VECTOR A + VECTOR B , we must understand vector is a quantity which has magnitude(value) + direction. so when we add vectors we have to add both values with direction that can not be done with simple addition. we need to use trignometry like pyhtagorus theorem, sin and cos rule to add vectors.
P.S.- if two vectors have same direction then we can get resultant vector by just adding values of two vectors. but if vectors have different directions then we need to use trignometry like pythagorus theorem.