# Calculating the distance from a point to a plane

• Darkmisc
In summary, the conversation discusses finding the distance between a point and a plane using the formula d=|PQ*n| and choosing a point on the plane. There is confusion about how to determine if a point is on the plane and whether a parallelogram can be formed without the point touching the plane. The solution provided has some errors and another approach is suggested.
Darkmisc
Homework Statement
What is the distance from r*(2i -j -2k) = 7 to the point (1, 1, -1)?
Relevant Equations
d=|PQ*n|
Hi everyone

I have worked solutions to the question, but I don't fully understand what they are doing and I get a different answer.

I used d=|PQ*n| and chose (0, 0, -7/2) as a point on the plane. I got that point by letting i and j = 0.

Since P = (1, 1, -1), PQ = (-1, -1, -5/2).

The unit vector of the normal works out to be (1/3)(2i -j -2k)

So d = |(-1, -1, -5/2)*(1/3)*(2, -1, -2)| = 4/3.

These are the worked solutions.

I understand the general logic behind it and can accept that OB' = BM. However, I don't understand how they know M is a point on the plane. Is it not possible for OB'MB to form a parallelogram without M touching the plane?Thanks

Darkmisc said:
I understand the general logic behind it and can accept that OB' = BM. However, I don't understand how they know M is a point on the plane. Is it not possible for OB'MB to form a parallelogram without M touching the plane?
You define M to be the point on the plane closest to B.

That said, the solution is full of mistakes. First, it should be ##\overrightarrow{OA} = \frac{7}{2} \mathbf{k}##.

Then, I get
$$\frac{\overrightarrow{OB} \cdot \mathbf{n}}{|\mathbf{n}|} = 1,$$
not ##1/3##, such that ##|\overrightarrow{OB'}| = 1##. Since ##|\overrightarrow{ON}| = 7/3##, this means that ##|\overrightarrow{B'N}| = 4/3##. I then take ##|\overrightarrow{BM}| = |\overrightarrow{B'N}| = 4/3##, so I get the same answer as you did (and I like your approach better). I'm not sure I get the parallelogram either.

Darkmisc

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