When dave solved the following question , His workings were as follows Question : Find the resultant of two vectors of 3 units and 4 units acting at a point O at an angle of 45° with each other.

Workings:(see image)

Main Question : Why did Dave change the Subtraction sign to addition sign ? (see 1st and 2nd line of workings) or see 2nd image.

Well, the lines of workings are cut off in the middle, and there's no 2nd image.

But I think it must have to do with the "cos" terms at the right end of the picture.

I'm guessing now, because the part I'm interested in would be just past the edge, where we can't see it.

I think that the first line says "cos(some angle)", and at the same place in the second line it says "cos(180 - the same angle)".

If that's what it says, then that's your answer, because cos(anything) is equal to the negative of cos(180 - the same thing).

That's the best I can do for you just now.  Honestly, I don't see the connection yet between the question Dave is working on and the two lines I see in the picture.

the answer is kevlar