Hi guys,

I'm having a problem with transposing the equation below to make the subject:

denotes the length of the vector as in

I know the trig identity where is equal to where and which would get rid of the two instances of .

However are all 3D vectors which is where I'm having the problem. I can't see how I can transpose the equation to make Vf the subject. Is there another identity I'm missing?

Any help is much appreciated.