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# C2 - Trig Question, cast diagram/othermethods Watch

1. Hey, I do edexcel A Level maths..
for C2 we haven't been taught how to use CAST diagram or the quadrant rule..
we have just been taught how to work out values of θ by using calculator values..

so for e.g. if θ = 0.5... then sin = 30
then you do 30 add 360 each time to find the values...
and then you do 180 - 30 (so 150) and then keep adding 360 to that value...

thats just for the positive side.. for negative side you keep subtracting 360..

Do we HAVE to know how to use CAST/quadrant rule for C2 edexcel maths or is this method appropriate????
2. To be honest whenever I've tried any method other than cast, it goes horribly wrong (especially if its something like 4x-30=sin(x)....)
However some of my friends do it the way you are talking about but my maths teacher told us that it can go wrong. Anyway hope this helps!
3. (Original post by Bhudsz)
Hey, I do edexcel A Level maths..
for C2 we haven't been taught how to use CAST diagram or the quadrant rule..
we have just been taught how to work out values of θ by using calculator values..

so for e.g. if θ = 0.5... then sin = 30
then you do 30 add 360 each time to find the values...
and then you do 180 - 30 (so 150) and then keep adding 360 to that value...

thats just for the positive side.. for negative side you keep subtracting 360..

Do we HAVE to know how to use CAST/quadrant rule for C2 edexcel maths or is this method appropriate????
I suppose that method is alright. Perhaps it would be easier if you familiarized yourself with the shapes of the trigonometric curves . That's what I've done. Then I just work out values based on their respective periodicities.

(just an alternative, you know)
Attached Images
5. cos, tan, sin.pdf (132.8 KB, 233 views)
6. I find that the best way for me is to draw a sketch of the graph sin(θ) or cos(θ) or tan(θ) and then think about it graphically. Then I don't have to remember all the rules, I can just figure them out from the graph.

Let's use the example of sin(θ)=0.5 by sketching the graph first.

Now if we calculate our initial value for θ we can plot this point on the graph, The co-ordinates being (θ, 0.5). I got θ = pi/6 or 30 degrees.

Now we can draw a horizontal line at 0.5 and all the points of intersection marked are also values of sin(pi/6). We can calculate them easily with some basic geometry.

Hope this helps
Attached Images

7. (Original post by Hasufel)

(just an alternative, you know)
that doesn't make much sense to me!
8. and ahhhh yeah that's what I mean, like as in, you picture the graph/draw it and draw a line and see where it meets the curve!
then you can work it out...
Is this way applicable to all questions though?

Because when I go on exam solutions or look in the textbook (the examples) they always use cast diagrams!
9. (Original post by Bhudsz)
and ahhhh yeah that's what I mean, like as in, you picture the graph/draw it and draw a line and see where it meets the curve!
then you can work it out...
Is this way applicable to all questions though?

Because when I go on exam solutions or look in the textbook (the examples) they always use cast diagrams!
It is applicable to all Qs. At least C1-4. I'm not sure about the FP modules
10. I've also disliked using cast diagrams and remembering the rules for how to use it. It seems overly complex compared to the graph-intersection method. After a while you don't have to draw the graph because you can calculate it in your head by picturing it. Then eventually you don't even have to picture it, you just know the pattern for calculating sin (x), cos (x) and tan (x). That's just my opinion but I suppose if you did the cast method enough times you'd get used to that too. So it could be a matter of preference, but the important thing is that they're both correct methods.
11. (Original post by hoodboilu4)
I've also disliked using cast diagrams and remembering the rules for how to use it. It seems overly complex compared to the graph-intersection method. After a while you don't have to draw the graph because you can calculate it in your head by picturing it. Then eventually you don't even have to picture it, you just know the pattern for calculating sin (x), cos (x) and tan (x). That's just my opinion but I suppose if you did the cast method enough times you'd get used to that too. So it could be a matter of preference, but the important thing is that they're both correct methods.
yeahhhh! that's exactly what I'm thinking I think I'm just going to carry on doing it our way, it seems much easier and there's no point in me learning a whole new method in calculating the values when we're this close to exams.. Might as well just get better at our method/way of doing it if they're both correct and perfectly fine

Thank you alllll! tsr is very helpful ^.^

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