# Thread: Magnitude of vectors question

1. ## Magnitude of vectors question

Hello, I'm very relieved to have found this website!!

I am trying to do the following question:-
A rectangle OBCA in the xy-plane has vertices O(0,0), B(-3,4), C(5,10) and A(8.6).

Determine the magnitude and direction of the following vectors

1) OB
2) OD
I have drawn this:-

I'm attempting part 1 and have the magnitude (which is 5) but am unsure how to work out the magnitude.

I have the answer (it is 126.87 degrees), but this is confusing me as the triangle looks to be a right angled triangle, and so 90 + 126.87 is way over 180 degrees, so I'm clearly not working this out correctly.

I've memorised SOC-CAH-TOA.

Many thanks!!!

2. Originally Posted by Cheryl W
Hello, I'm very relieved to have found this website!!

I am trying to do the following question:-
I have drawn this:-

I'm attempting part 1 and have the magnitude (which is 5) but am unsure how to work out the magnitude.

I have the answer (it is 126.87 degrees), but this is confusing me as the triangle looks to be a right angled triangle, and so 90 + 126.87 is way over 180 degrees, so I'm clearly not working this out correctly.

I've memorised SOC-CAH-TOA.

Many thanks!!!
The angle formed in your right triangle at the vertex of OB and the x-axis is arctan(4/3) or approx. 53.13 degrees.

The angle that OB (terminal side) makes with the x-axis (initial side) is 180-53.13 or approx. 126.87 degrees.

3. Originally Posted by Cheryl W
Hello, I'm very relieved to have found this website!!

I've memorised SOC-CAH-TOA.

Many thanks!!!
Actually, it's SOH-CAH-TOA

4. Thank you masters, figured it out now.

Will try a few more for practice.