# FIND THE MAGNITUDE AND DIRECTION COSINES OF GIVEN VECTORS

## About "Find the Magnitude and Direction Cosines of Given Vectors"

Find the Magnitude and Direction Cosines of Given Vectors :

Here we are going to see how to find the magnitude and direction cosines of given vectors.

## Find the Magnitude and Direction Cosines of Given Vectors - Practice Question

Question 1 :

If

find the magnitude and direction cosines of

(i) a vector + b vector + c vector

(ii)  3a vector - 2b vector + 5c vector

Solution :

(i) a vector + b vector + c vector

=  (2i + 3j - 4k) + (3i - 4j - 5k) + (-3i + 2j + 3k)

=  (2i + 3i - 3i) + (3j - 4j + 2j) + (-4k - 5k + 3k)

=  (2i + j - 6k) vector

|a vector + b vector + c vector|   =  √22 + 12 + (-6)2

=  √(4+1+36)  =  √41

Direction cosines are (x/r, y/r, z/r)

That is, (2/√41, 1/√41, -6/√41)

Hence magnitude and direction cosines are √41 and (2/√41, 1/√41, -6/√41) respectively.

(ii)  3a vector - 2b vector + 5c vector

Solution :

3a vector  =  3(2i+3j-4k)-2(3i-4j-5k)+5(-3i+2j+3k)

=  (6-6-15)i + (9+8+10)j+(-12+10+15)k

=  -15i + 27j + 13k

|3a vector - 2b vector + 5c vector  =  √(-15)2 + 272 + 132

=  √225 + 729 + 169

=  √1123

Direction cosines are (x/r, y/r, z/r)

That is, (15/√1123, 27/√1123, 13/√1123)

Hence magnitude and direction cosines are √1123 and (15/√1123, 27/√1123, 13/√1123) respectively.

Question 2 :

The position vectors of the vertices of a triangle are i+2j +3k; 3i − 4j + 5k and − 2i+ 3j − 7k . Find the perimeter of the triangle (Given in vectors)

Solution :

To find the perimeter of the triangle, we have find the sum of all sides.

OA vector  =  i + 2j + 3k

OB vector  =  3i − 4j + 5k

OC vector  =   -2i+ 3j − 7k

AB  =  OB - OA

=  (3i − 4j + 5k) - (i + 2j + 3k)

AB vector  =  2i - 6j + 2k

|AB vector|  =  √22 + (-6)2 + 22

=  √(4+36+4)  =  √44  ----(1)

BC  =  OC - OB

=  (-2i+ 3j − 7k) - (3i − 4j + 5k)

BC vector  =  -5i + 7j - 12k

|AB vector|  =  √(-5)2 + 72 + (-12)2

=  √(25 + 49 + 144)  =  √218  ----(2)

CA  =  OA - OC

=  (i + 2j + 3k) - (-2i+ 3j − 7k)

CA vector  =  3i - j + 10k

|AB vector|  =  √32 + (-1)2 + 102

=  √(9 + 1 + 100)  =  √110  ----(3)

Perimeter of triangle  =  √44 + √218 + √110

After having gone through the stuff given above, we hope that the students would have understood,"How to Prove the Given 4 Vectors are Coplanar"

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