# Homework Help: Physics formula for vectors

1. Sep 12, 2009

### tnutty

1. The problem statement, all variables and given/known data

Find the area of the parallelogram with the vectices :

A(-2,1)
B(0,4)
C(4,2)
D(-2,1)

2. Relevant equations

This sections is about vectors, specifically crossproducts.

A = |a|(|b|sin(/theta) = |a x b|

- The length of the cross product a x b is equal to the area of the parallelogram determined
by a and b

3. The attempt at a solution

Instead of using the equation i found the length of each line :

A to B = sqrt(13)
B to C = sqrt(20)
D to C = sqrt(14)
A to D = sqrt(20)

What I did was divide it into 2 triangles find each hypotenuse and multiply it by each other
then divide it by 2. I don't know if there is a formula similar to this but the answer I got
was 16.748.

The problem I am having is to use the definition in the Relevant equation sections above.
How do I know which points would be a vector. If I know that then I can
probably figure about there Cross product and find its magnitude which is the parallelogram area.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 12, 2009

### VeeEight

3. Sep 12, 2009

### tnutty

Re: Vectors

Oh I see, All I need is x and y : x*y = Area, where x & y are vectors.

But will it always be the case that the length of the A to D will be equal to the length B to C?

4. Sep 12, 2009

### VeeEight

Re: Vectors

Remember to take the magnitude of the cross product (i.e |x*y| )

No.

5. Sep 12, 2009

### tnutty

Re: Vectors

How do I counteract that if it does not have the same length on the opposite side.

Do I have to do 2 cross product?

6. Sep 12, 2009

### HallsofIvy

Re: Vectors

Opposite sides in a parallelogram are always of equal length.

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