Thread: Complex vector question

Hi, here's a question from an exam that I don't know how to do. I can do (i) and (ii). For (ii) I get or . But I don't know what they want for (iii) and (iv).

I was thinking of using the fact that the diagonals of a paralleogram bisect each other, but that doesn't seem to work out...

(i) Let and . If and are parallel then prove that for k real.

(ii) Let the points A, B, C and D be represented by the complex numbers , , and respectively (in clockwise order). Find two possible vectors representing side AB.

(iii) If A, B, C and D are the vertices of a parallelogram then using parts (i) and (ii) find an expression for side DC in terms of AB using the vectors , , and .

(iv) Using a property of a parallelogram find the two possible values of k.

(v) Show that for both these values of k that .

Any ideas?

Thanks

EDIT: Does this belong in the pre calculus section? If so, many apologies. My university considers complex numbers as an algebra topic...

Some of these problems seem strange.

For (iii), the vector representing the side DC is equal to the vector sum DA + AB + BC, which I guess is an expression using AB and the four given vectors.

(iv): Hm? We know that the two vectors and are parallel. Doesn't this mean that the points A and B lie on the same straight line through the origin? I don't see how this restricts the value of k in any way.

Hello,

Sorry I made a typo the line in the last part should be " ."

Thanks again

Originally Posted by HappyJoe

Some of these problems seem strange.

For (iii), the vector representing the side DC is equal to the vector sum DA + AB + BC, which I guess is an expression using AB and the four given vectors.

(iv): Hm? We know that the two vectors and are parallel. Doesn't this mean that the points A and B lie on the same straight line through the origin? I don't see how this restricts the value of k in any way.

I think it's hinting at the fact that sides DC and AB are parallel, and I need to use that result somehow...

Originally Posted by minifhncc

I think it's hinting at the fact that sides DC and AB are parallel, and I need to use that result somehow...

As I see it, the vectors and don't have anything to do with the side DC, do they? They represent the point A and B, and for the two vectors to be parallel, the two points A and B have to lie on the same straight line through the origin.

I think it means to take it generally.

ie. and in part (i) aren't the same vectors in the following parts.

Originally Posted by minifhncc

I think it means to take it generally.
ie. and in part (i) aren't the same vectors in the following parts.

From reply #2, “Some of these problems seem strange”. I absolutely agree with that. Moreover, whoever wrote certainly knows what he/she means. But that does not mean that we do. That said, here are some observations.

If are the vertices of a parallelogram in counter-clockwise order the side can be represented as the vector .

Because we have a parallelogram, it must be true that
.

Does that help?

Originally Posted by Plato

Because we have a parallelogram, it must be true that
.

Does that help?

Yes I know that the diagonals of a parallelogram bisect each other... but how do I get the two values of k ...? Wouldn't that expression only give one value of k?

Thanks

Originally Posted by minifhncc

Yes I know that the diagonals of a parallelogram bisect each other... but how do I get the two values of k ...? Wouldn't that expression only give one value of k?

Well, my reply has absolutely nothing to do with the diagonals of a parallelogram.

We want to find k in , is that right?

In part (iv), what is your interpretation of and , that is, which part of the parallelogram do they represent?

and , I think, are just two arbitary vectors.

ie. I'm thinking that since sides AB and DC are parallel, we need to find k such that . Upon solving it I get . But for both these values of k I don't get the required expression. I only get it for one value ie. k=-1...

Originally Posted by minifhncc

and , I think, are just two arbitary vectors.

ie. I'm thinking that since sides AB and DC are parallel, we need to find k such that . Upon solving it I get . But for both these values of k I don't get the required expression. I only get it for one value ie. k=-1...

This is what Plato said.

You are right that for a pair of parallel vectors and , there exists only one , such that , so them asking for two such values of is strange.

Unless of course they also want a such that , where we are looking at sides and , which they just might do, even though the problem text takes care not to give anything away.