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# P3 vector plane angle, helppppp. Watch

1. Hi,

When we try to find out the angle,x, between two planes, how do we know if we need to subtract 90 degree from the answer theta or not. Sometimes, we subtract the answer from 180 degree.

The question is:
n1 = i + 2j - 2z, n2= 2i + j + 3z

Now find the angle between the two. But how?
Plz help me out...
thanks for trying to help....
2. (Original post by methewthomson)
Hi,

When we try to find out the angle,x, between two planes, how do we know if we need to subtract 90 degree from the answer theta or not. Sometimes, we subtract the answer from 180 degree.

The question is:
n1 = i + 2j - 2z, n2= 2i + j + 3z

Now find the angle between the two. But how?
Plz help me out...
thanks for trying to help....
Use the dot product.

then you'd want the acute angle.

3. Planes will always meet in two supplementary angles. The question will usually ask for the acute angle, in which case you do 180 minus if and only if the angle you found is obtuse.

You do not need to do 90 minus for planes. However, you do when you're finding the angle between a line and a plane. This is because the angle between the line and the normal is NOT the angle between the line and the plane but its complement.
4. Thank you both very, very, very much for explaining the seemingly inexplicable...
You both have been very helpful...

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