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# CAST diagram trig. Watch

1. When doing trig. questions the CAST diagram method, you have to find out if the value is positive or negative in order to know which quadrants to use. Can someone tell me how to figure out whether it is positive or negative? I don't know whether to use the angle or the answer of the sin/cos/tan(x)=...
2. You seem confused. I suppose that's why you're asking the question .

Post a question and your attempt to answer it and someone will be happy to help.
3. It's the other way around:

CAST is way of quickly visualising whether the sin/cos/tan of any given angle is a +ve or -ve value.
4. When you get a question like tanx=tan(-120[degrees]), here x=-120[degrees]. The next step is to find out which quadrants to use on the cast diagram. But you have to find out if it is positive or negative and this is the part I am confused on. I don't know which number to look at for the positive or negative. The 120[degrees] or the answer to tan(120[degrees]) which equals to sqr. root 3 and this is positive. So I have a positive and negative value here. Sorry i cant explain this properly if you get confused
5. (Original post by raptor12)
When you get a question like tanx=tan(-120[degrees]), here x=-120[degrees]. The next step is to find out which quadrants to use on the cast diagram. But you have to find out if it is positive or negative and this is the part I am confused on. I don't know which number to look at for the positive or negative. The 120[degrees] or the answer to tan(120[degrees]) which equals to sqr. root 3 and this is positive. So I have a positive and negative value here. Sorry i cant explain this properly if you get confused
So the -120 degrees is actually: 360 - 120 = 240 degrees. (every multiple of 360 is the same. i.e. 0, 360, 720, 1080, 1440 etc)

Use the CAST with 240 degrees which falls in the 3rd quadrant, i.e. casT where only the Tangent is +ve. Viola.

So in your case you have Tan(-120) = Tan(360 -120) = Tan(240)

240 is in the 3rd quadrant and so Tan(-120) is +ve.

Addendum: the idea is to always get your angle first as a +ve value beween 0 and +360. When you have done that, then and only then use the CAST system to find out whether the sin/cos/tan of the angle is +ve or -ve.
6. (Original post by raptor12)
When you get a question like tanx=tan(-120[degrees]), here x=-120[degrees]. The next step is to find out which quadrants to use on the cast diagram. But you have to find out if it is positive or negative and this is the part I am confused on. I don't know which number to look at for the positive or negative. The 120[degrees] or the answer to tan(120[degrees]) which equals to sqr. root 3 and this is positive. So I have a positive and negative value here. Sorry i cant explain this properly if you get confused

Does that quadrant have positive or negative tangent

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Updated: May 12, 2013
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