Posted by Trig on Tuesday, November 3, 2009 at 8:27pm.
How come cosine of 225 degrees is a positive number i thought it was negative because it's reference angle would be 45 degrees below the second quadrant and in a unit circle cosine is just the x value so I thought cosine of something in both the second and third were negative
apparently i'm wrong please explain
- Trig - Reiny, Tuesday, November 3, 2009 at 8:33pm
225º is in the fourth quadrant, 45º below the first quadrant, so its x value is indeed positive.
Check how angles are measured.
- Trig - Trig, Tuesday, November 3, 2009 at 8:34pm
I think i was in radians my bad
um how come tan of 300 is negative root three
tangent in unit circle is y/x I am getting .5/(root three/2)
which is not root three
- OOPS -Trig - Reiny, Tuesday, November 3, 2009 at 8:36pm
Forget about my previous post.
Obviously 225º is in the third quadrant, 45º below the second quadrant like you said.
So of course cos 225 is negative.
- Trig - Reiny, Tuesday, November 3, 2009 at 8:38pm
= -tan 60º by the CAST rule
= -√3/1 from the ratios of the 30-60-90 triangle
- Trig - Trig, Tuesday, November 3, 2009 at 8:44pm
why wouldn't you use the 30 degree angle?
I did 360 -90 to get 270 degrees and then did 300 - 270 to get 30 degrees and used that angle why would I use the 60 degree angle instead of the thirty
- Trig - Reiny, Tuesday, November 3, 2009 at 9:03pm
Always ask yourself,
"How far away from the x-axis am I "?
to get the reference angle.
e.g. sin 160
160º is 20º away from the x-axis and according to the CAST rule in quadrant II the sine is positive, so
= sin 20º
e.g. tan 310º
310º is 50º from the x-axis and according to the CAST rule, in quadrant IV the tangent is negative, so
= -tan 50
check my examples with your calculator
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