# 2-forms

• Nov 10th 2007, 11:45 AM
shilz222
2-forms
Find the value of the 2-form $\displaystyle dx \ dy + 3dx \ dz$ on the oriented triangle with vertices $\displaystyle (0,0,0), \ (1,2,3), \ (1,4,0)$.

I drew the projection of the triangle on each of the coordinate planes. Now what do I do? Do I compute the area of the triangle on the $\displaystyle xy$ plane?

Because $\displaystyle \text{flow} = A dy \ dx + B dz \ dx + C dx \ dy$
• Nov 10th 2007, 12:21 PM
shilz222
Is the basically the sum of the areas in the projected planes?
• Nov 10th 2007, 12:34 PM
shilz222
The answer is $\displaystyle -3 \frac{1}{2}$. And if it is a 2 form then it has to be assigned a number to a oriented surface. Maybe I should draw a prism?
• Nov 10th 2007, 12:55 PM
shilz222
I think I got it. Its $\displaystyle \left(\text{area of projection on xy axis} \right) + \left( 3 \times \text{area of projection on xz axis} \right)$.

I ended up getting $\displaystyle - \left( \frac{18}{4} - \frac{\sqrt{14}}{4} \right) \approx -3.5$
• Nov 10th 2007, 01:52 PM
shilz222
But if the surface is not a triangle then you can't do this right?