# Find the Dimension

Discussion in 'Math' started by lomax1, Mar 9, 2014.

1. ### lomax1 Thread Starter New Member

Feb 6, 2014
27
0
Can someone assist me with a walk through for this please?

A design brief indicates three holes A, B and C to be drilled in a plate, with a = 73mm, b = 117mm and A = 37 Degrees

 Determine the possible values for the dimension c.

2. ### WBahn Moderator

Mar 31, 2012
20,057
5,651
As you've been told many times, you need to show YOUR best attempt to solve YOUR homework problems. So put together a sketch and then show your attempt. That will give us a starting point for discussion.

3. ### pwdixon Member

Oct 11, 2012
490
56
With the data provided the best I can do is say 42 and that's just because it's the first number that comes to mind. Is there more to this question?

4. ### lomax1 Thread Starter New Member

Feb 6, 2014
27
0
no that is all I am afraid, this is why I am stuck. could you plesae explain how you got to 42 please?

5. ### pwdixon Member

Oct 11, 2012
490
56
This is obviously some kind of wind-up.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
This question is obviously about the ambiguous case of the sin rule for plane triangles.

However it is extremely poorly worded.

and nothing else.

Then I can only suggest you get a better teacher asap.

What did the teacher actually ask?

Incidentally, in the machine shop they would answer this with a scriber, ruler, protractor and scribing compass.

If you would like to explain what you think the question means, and in particular include everything that was in it,
we can go through it step by step.

You can obtain a lot of first class help here (and many have), but it is important to cooperate with the helpers.
It is no use saying "pretty please do it for me"

But at the moment you are saying nothing at all about what do you know, so I don't know where to start you off.

So give me an idea of what you think the problem might be about or what you don't understand or something.
So is this a workshop question, a trigonometry question, a geometry question or what?
It doesn't matter if that something is right or wrong it is very useful to the helper.

Last edited: Mar 12, 2014
7. ### lomax1 Thread Starter New Member

Feb 6, 2014
27
0
This is clearly a Trigonmetric question and yes that is simply all that has been put.

Now as I understood like triangles this could be split into two sections Top and bottom?
I could apply the sine rule to work out the points which is relatively easy however I wouldn't know how to convert this into degrees?

8. ### Metalmann Active Member

Dec 8, 2012
700
224
"A design brief, indicates three holes A, B and C to be drilled in a plate, with a = 73mm, b = 117mm and A = 37 Degrees

 Determine the possible values for the dimension c."

You need a reference surface first.

9. ### lomax1 Thread Starter New Member

Feb 6, 2014
27
0
That is all I have, I also have 'Higher Engineering Mathematics' by John Bird issued to me by my tutor, however there isn't even a similiar example in there either.

10. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
Although you have not responded to what has been asked of you I will have one more try to help.

The question tells you that there could be more than one value for dimension c

WBahn asked you to draw a diagram.
Have you drawn one?
If you have difficulty posting a diagram, let us know since that can be overcome.

I have drawn a series of steps to understand the problem.

Step (1) I have drawn a standard triangle, and labelled it in the standard manner.

Note that the points (vertices) are labelled with upper case letters and I have shown the angle sign for the angles since they also use upper case letters.
Lengths are labelled with lower case letters and if you are not familiar with the placement of the sides and angles you should learn them carefully.

This is important since A is a point not an angle as stated in your rendering of the question.

I have also listed the known information here.

Step (2) I have started from point A and drawn a line that point B lies on.
We do not yet know where B is just that it lies on the line.

Step (3) We are told that angle A is 37° so I have drawn another line at 37° from point A.
Point C lies on this line.

Step (4) We are told that AC = length b = 117mm so I have marked off point C along the line at this distance.

Step (5) We are told that CB = length a = 73mm.
So I set my compasses to this distance and draw a (part) circle centred on C.
This cuts the line from A to B at two places.
I have called these B1 and B2

This means that I can draw two different triangles with the given information, so can find two different distances for c.
I have called these c1 and c2.

Both of these can be measured on my diagram.

So far this has been a question of geometry, not trigonometry.

When you have responded to the story so far we can talk about how the issue is resolved when we calculate c instead of measuring it.

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Last edited: Mar 13, 2014
panic mode and Papabravo like this.
11. ### WBahn Moderator

Mar 31, 2012
20,057
5,651
What is so difficult about putting together a drawing showing what the dimensions you have given mean? Why does that appear to be an unreasonable thing to ask?

12. ### panic mode Senior Member

Oct 10, 2011
1,407
344
where is this "design brief"? show picture... then explain what you think question asks and what path toward solution should be.

13. ### panic mode Senior Member

Oct 10, 2011
1,407
344
in a movie "Hitchhiker's guide to the galaxy" computer was asked to answer universal question "what is the meaning of life". after computing for millions of years, computer eventually came up with an answer:

42

14. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
For the benefit of those who liked my geometry, this corresponds to the ambiguous case in the sin rule.

The ambiguity arises because one angle in a triangle can be larger than 90°, if the other two are small enough.

So if we know one small angle and two sides the formula

$\sin B = \frac{{b\sin A}}{a}$

returns two possible values for angle B.

One is inthe first quadrant ie < 90°
One is in the second quadrant ie > 90°

since the sine is positive in both these quadrants.