# Homework Help: Vectors in 3 space

1. Oct 4, 2005

### blimkie

calculate the angles bewteen of the intersection of the planes:
2x+y-3z+7=0 and 4x-y+7z+5=0

any takers? i just need an idea of where to start

thanks

2. Oct 5, 2005

### HallsofIvy

Do you know how to find the normal vectors for the two planes?
Do you know how to find the angle between two vectors?

Do you see why the angle between the two vectors is the angle between the planes?
(Oh, and there is only one angle, not "angles".)

3. Oct 5, 2005

### TD

Well there is always the supplementary angle too of course, but if you have one, the difference between pi and that angle is the other one.

4. Oct 5, 2005

### amcavoy

Just to add:

$$a(x-x_0)+b(y-y_0)+c(z-z_0)=0$$

is the general equation of a plane. The vector <a,b,c> is the normal vector to that plane. Can you see where to get a, b, and c from your original planes?

Alex

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