In the half-angle identities, can someone please explain the plus-minus-symbol? When do you change it to a +? A negative? See this page for the the identities that I am confused about: http://bit.ly/z3O5ki

Question

mertsj · Answer

You choose the positive or negative depending on the sign of the function of the given angle

anonymous · Answer

? Have you visited that website? That's what I am confused about those plus/minus signs

mertsj · Answer

Do you mean why are the plus/minus signs there?

anonymous · Answer

Yes, like in the final answer do you use the +? or the -?

mertsj · Answer

You choose the positive or negative depending on the sign of the function of the given angle

mertsj · Answer

Do you have a specific problem?  I will help you figure out the sign.

anonymous · Answer

Well, I just want to know a rule or something because It'll show up on the test and I want to know which to use. Is it something to do with ASTC (when the angles are positive or something? )

anonymous · Answer

ASTC - All (Q1) students (Q2) take (Q3) calculus (Q4)

anonymous · Answer

When the trig ratios are +

anonymous · Answer

in what quadrant

mertsj · Answer

The only rule I know is the the cos is like x so it is positive in Q1 and Q4
The sin is like y so it is positive in Q1 and Q2
The tan is sin/cos so it is pos in Q1 and Q3

mertsj · Answer

And that is the way you choose the plus/minus in the half angle formulas...based on the quadrant of the angle.

anonymous · Answer

Okay that makes sense. so the +/- is based on what Quadrant the angles are +.

phi · Answer

Here is an example

find the tan(240º/2)    that is , tan(120º) = ±sqrt( 1-cos(240) / (1+cos(240) )= ±sqrt(3)
120º is in the 2nd quadrant, so you pick the minus value:  -sqrt(3)
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