# Cosine Rule , what good is it ?

1. Oct 16, 2006

### junior_J

Cosine Rule , what good is it ????

Does the Cosine rule hold true for say negative lengths ??? as in a vector quantity like displacement ???

I came across this problem which had -15km and 10km as the known sides whereas the angle opposite the unkown side is 60 degree ....

I tries using c^2 = a^2 + b^2 - 2*a*b cosC but this didnt work .... then on account of frustration i forcibly changed the '-' part of the formula to a '+' only to find that it works ... ???

Can someone explain this to me ??

2. Oct 16, 2006

### Zelos

ther is no such thing as negative lenght, there is negative coordinates since somepoint has to be 0 and one side will be posetive and the other negative

3. Oct 16, 2006

the 60 degrees is negative, because it is east of north.

4. Oct 16, 2006

### junior_J

negative displacement ????? would that apply to the cosine rule ?

5. Oct 16, 2006

### HallsofIvy

There is no such thing as a negative length. A vector quantity (say, in a one-dimensional problem on a number line), or the components of the vector might be negative but the length is always non-negative. If you are using the cosine law in a vector problem, use the lengths, not the components.

6. Oct 16, 2006

### junior_J

so if i had a velocity oh say -15 km/hr ..... id use 15 instead of -15 ?? as one of the sides of a triangle ???

7. Oct 16, 2006