Vectors Using Cosine or Sin Law

[xChee]

Homework Statement

Need help with a question ... .__."

q: if the magnitude of a=2 and the magnitude of 5a-2b= 7.7, and the angles between the vectors a and b is 50 degrees determine the magnitude of b.

Homework Equations

The Attempt at a Solution

first I drew a diagram... then i used cosine law to solve for b

l5a-2bl = (5x2) + (2b)^2 -2(5x2)(2b)cos50

7.7= 10 +4b^2 -40b cos 50

this worked out to be a quadratic, the zeros i found were 6.3 & 0.09, but then i subbed those numbers back into the cosine law i don't get back 7.7...

I tried this question like this, l5a-2bl = (5x2)^2 + (2b)^2 -2(5x2)(2b)cos50, but it didn't work out either.

help?

 

[e^(i Pi)+1=0]

Why not just use algebra to solve for b?

 

[azizlwl]

Homework Statement

Need help with a question ... .__."

q: if the magnitude of a=2 and the magnitude of 5a-2b= 7.7, and the angles between the vectors a and b is 50 degrees determine the magnitude of b.

Magnitude of vectors are scalar quantity. Operations just like ordinary real numbers.