Need help with trig!

[astone788]

Need to find the angle CNT

From the diagram it's clear that angle CNT is an obtuse angle. However the actual answer is 69 degrees. Wtf? Because we're using the Sine rule to find out an obtuse angle why the hell doesn't the ambiguous case of the sine rule apply?  Hence the answer should be 180-69. Wtf?

Perhaps the drawings are totally inaccurate, but that should be inexcusable during this time of year.

[Will T]

Let

Let

[StumbleBum]

Need to find the angle CNT

From the diagram it's clear that angle CNT is an obtuse angle. However the actual answer is 69 degrees. Wtf? Because we're using the Sine rule to find out an obtuse angle why the hell doesn't the ambiguous case of the sine rule apply?  Hence the answer should be 180-69. Wtf?

Perhaps the drawings are totally inaccurate, but that should be inexcusable during this time of year.

How is this a case of the ambiguous sine rule..? Before you can use the sin rule you need to find the third side length, which you use the other values to calculate. Once you've done this you then use the sine rule, for which there can only be one possible value as you now have all three sides...

And as will just demonstrated, doing it his way will yield only one possible answer aswell..

[astone788]

sine rule can only calculate acute angles. Looking at the diagram angle CNT is an obtuse angle. Therefore to get the obtuse angle you do 180 - acute angle (69)

[StumbleBum]

sine rule can only calculate acute angles. Looking at the diagram angle CNT is an obtuse angle. Therefore to get the obtuse angle you do 180 - acute angle (69)

Need I remind you what the first page of the exam states "Diagrams are not to scale unless specified otherwise", you can't assume an angle is obtuse from looking at the diagram. Despite the fact that it doesn't look obtuse anyway....

[astone788]

ta for your help. Not the first time this is happened. VCAA's diagrams are no way near accurate, yet they expect our diagrams to be perfect. bastards.

[astone788]

teacher told me, and in your maths book.
If the unknown angle is an obtuse angle, remember the following:

    the inverse sine function on calculators evaluates only the acute angle
    for the obtuse angle, evaluate as follows: obtuse angle = 180° − acute angle
    the ambiguous case of the sine rule occurs when the smaller known side is opposite the known angle.

[astone788]

by the way, I've done vcaa 06, 08-11 and I have not seen 1 question that test's your ability to recognize the ambiguity law. If anything I thought this question would test our ability. But chances are they wont test us on a bullshit ambiguous case question. Truth is you can't be tested. Their diagrams are inaccurate.

[sheep94]

It's not an obtuse angle. It just looks like it from this angle.

[astone788]

how do you tell?

[astone788]

Finished all the vcaa exams and not a single question was about the ambigious case of sine rule. My prediction is it wont be on tommorows exam, but you never know.

[sheep94]

Finished all the vcaa exams and not a single question was about the ambigious case of sine rule. My prediction is it wont be on tommorows exam, but you never know.

You can tell because if point T was moved to 090 deg from point N, it would be a few metres north from the tree, but since point T is at the tree, it has to be less than 90 degress.

[Mr Newton]

Look at it from a birds eye view, you can clearly tell it's acute

[Conic]

Again, the confusion is most likely due to the lack of scaling in VCAA's diagram and the angle that the situation is being view from. Here's a bird's eye view: