Trig help... please? :3 - Empty Closets - A safe online community for gay, lesbian, bisexual, transgendered people coming out

Austin · 15th Nov 2011, 09:55 PM

I know someone asked for calc help but now i rlyyy need some trig help...

For Question 8, I can't seem to understand what she wants. Notice that sin = -1/2... but sin is in Q2, right? ISn't tat impossible? Did she make a mistake?

For Question 9, I can't figure out how you know which quadrant 2theta is in? Because if you do 90<x<180 you get 180<2x<360... which is Q 3 and 4.... I mean, hmm... idk...

Sunandmoon · 15th Nov 2011, 09:58 PM

Picture didn't upload properly, at least for me

Austin · 15th Nov 2011, 10:02 PM

By the way, I didn't get to use a calculator on this test. So I don't get how you do it!

My friend helped me and we do the inverse of tangent to get it in Q4, but when you do it like 2tanx / 1-tan^2(x) you get a positive?? :s

---------- Post added 15th Nov 2011 at 10:19 PM ----------

Is it working now?

vyvance · 15th Nov 2011, 10:41 PM

Picture is showing now yeah.

Haven't taken trig since freshman year in high-school. Gonna take me a little while to remember it, but I'll try and dig through my notes.

Mad Man L · 15th Nov 2011, 10:46 PM

B is correct.

sin (theta) = 20/29
(theta) = sin^-1 (20/29)
(theta) = 43.6 degrees.

HOWEVER, angle is in second quadrant. i.e. 136.4 degrees. And go from there.

East stuff, I do Maths C. Ladies and men, please form a y = mx + c.

Austin · 15th Nov 2011, 10:54 PM

What I do not understand is, if you find tangent based on sin=20/29 (like she showed us how to do), you get tanx = 20/21... which when you plug into 2tanx/(1-tan^2(x))

Is 2(20/21) / (1-(20/21)^2)

Which is.... (40/21) / (441/441 - 400/441)

= (840/441) / (41/441) = 840/41

But that's postive...

Wait..... tanx should be -20/29....

which makes it negative... okay... i get it :S

But for number 8 she made an error right?

Mad Man L · 16th Nov 2011, 12:28 AM

There's a typo in Question 8.

3(pi)/2 < A < (pi)
Means:
3(pi)/2 < (pi)
Which is obviously wrong.

Assuming they meant this:
(pi) < A < 3(pi)/2, you get the angle of 210. (Working in degrees here, radians look ugly).
B is around 165.52 degrees (cba exact value), but all in all you get sin 44.48 = 0.7006. Subtract this from D, and it's pretty much correct.

Austin · 16th Nov 2011, 01:52 AM

Oh! Thanks for pointing that out... for some reason I kept getting 3pi/2 as 135 ugg idk why hahaha. So it's 270? So it's in Quadrant 3..... i think i asked her this when we were taking the test, I said "Is this right...?" And she said yes O_o but guess she didn't get what I was saying lol. So apparently I was not thinking for either of these two problems.

I guess I'll have to redo that... :P

not sure what u mean by

"Subtract this from D, and it's pretty much correct."

Thanks so much for the help guys!

Doctor Faustus · 16th Nov 2011, 02:48 AM

Was about to reply, but glad it was sorted out! Need algebra and lit help? (points)

caughtbywitness · 16th Nov 2011, 03:29 AM

Just took my maths exam. FML.

Austin · 16th Nov 2011, 03:52 AM

Quote:

Originally Posted by caughtbywitness

Just took my maths exam. FML.

How'd you do? :P On this test, the average was a 54... The highest was an 88. I think/thought I did decent. She said she's not giving us our scores back until thursday, and if we want we can complete the test for an extra 15 points if we get them all correct when re-doing it. :/

Quote:

Originally Posted by Doctor Faustus

Was about to reply, but glad it was sorted out! Need algebra and lit help? (points)

Haha thanks anyways!

I'll let you know!

Mad Man L · 16th Nov 2011, 04:26 AM

Quote:

Originally Posted by Austin

Oh! Thanks for pointing that out... for some reason I kept getting 3pi/2 as 135 ugg idk why hahaha. So it's 270? So it's in Quadrant 3..... i think i asked her this when we were taking the test, I said "Is this right...?" And she said yes O_o but guess she didn't get what I was saying lol. So apparently I was not thinking for either of these two problems.

I guess I'll have to redo that... :P

not sure what u mean by

"Subtract this from D, and it's pretty much correct."

Thanks so much for the help guys!

I'm talking about Option D in multiple choice (i.e. getting an exact value without having to deal with surds etc.)

Ionless · 18th Nov 2011, 07:46 AM

Not sure if you learnt this but ...

sin (A+B) = sinAcosB + cosAsinB
sin (A-B) = sinAcosB - cosAsinB
cos (A+B) = cosAcosB - sinAsinB
cos (A-B) = cosAcosB + sinAsinB
tan (A+B) = (tan A + tan B)/(1-tanAtanB)
tan (A-B) = (tan A - tan B)/(1+tanAtanB)

Exactly how these identities are derived I cannot remember anymore haha.

But using sin(A-B) = sinAcosB - cosAsinB and using a right angled triangle and assuming that it is 3pi/2 > A > pi,

sin(A-B) = (-1/2)(-sqrt 15 /4) - (-sqrt 3 /2)(1/4)
= sqrt 15 /8 + sqrt 3 /8
= (sqrt 15 + sqrt 3) / 8

which is option D

15th Nov 2011, 09:55 PM	#1
Austin Full Member Gender: Male Orientation: Gay Out Status: Most people in my life. Location: Orange County, California, USA Age: 19 Posts: 1,790 Join Date: Apr 2008	Trig help... please? :3 I know someone asked for calc help but now i rlyyy need some trig help... For Question 8, I can't seem to understand what she wants. Notice that sin = -1/2... but sin is in Q2, right? ISn't tat impossible? Did she make a mistake? For Question 9, I can't figure out how you know which quadrant 2theta is in? Because if you do 90<x<180 you get 180<2x<360... which is Q 3 and 4.... I mean, hmm... idk... __________________ Can I sail through the changing ocean tides, can I handle the seasons of my life? Last edited by Austin; 15th Nov 2011 at 09:58 PM..

15th Nov 2011, 09:58 PM	#2
Sunandmoon Member Regular Member Gender: Male Orientation: Gay Out Status: Family and close friends Location: Waterloo, Ontario Age: 21 Posts: 36 Join Date: Nov 2011	Re: Trig help... please? :3 Picture didn't upload properly, at least for me

15th Nov 2011, 10:02 PM	#3
Austin Full Member Gender: Male Orientation: Gay Out Status: Most people in my life. Location: Orange County, California, USA Age: 19 Posts: 1,790 Join Date: Apr 2008	Re: Trig help... please? :3 By the way, I didn't get to use a calculator on this test. So I don't get how you do it! My friend helped me and we do the inverse of tangent to get it in Q4, but when you do it like 2tanx / 1-tan^2(x) you get a positive?? :s ---------- Post added 15th Nov 2011 at 10:19 PM ---------- Is it working now? __________________ Can I sail through the changing ocean tides, can I handle the seasons of my life?

15th Nov 2011, 10:41 PM	#4
vyvance Well Known Regular Member Gender: Male Orientation: Gay Location: Texas Age: 23 Posts: 234 Join Date: Nov 2011	Re: Trig help... please? :3 Picture is showing now yeah. Haven't taken trig since freshman year in high-school. Gonna take me a little while to remember it, but I'll try and dig through my notes.

15th Nov 2011, 10:46 PM	#5
Mad Man L EC Addict Full Member Gender: Male Orientation: Bisexual Out Status: All but family Location: Brisbane, Australia Age: 16 Posts: 1,244 Join Date: Jul 2011	Re: Trig help... please? :3 B is correct. sin (theta) = 20/29 (theta) = sin^-1 (20/29) (theta) = 43.6 degrees. HOWEVER, angle is in second quadrant. i.e. 136.4 degrees. And go from there. East stuff, I do Maths C. Ladies and men, please form a y = mx + c. __________________ I really should get a proper signature...

15th Nov 2011, 10:54 PM	#6
Austin Full Member Gender: Male Orientation: Gay Out Status: Most people in my life. Location: Orange County, California, USA Age: 19 Posts: 1,790 Join Date: Apr 2008	Re: Trig help... please? :3 What I do not understand is, if you find tangent based on sin=20/29 (like she showed us how to do), you get tanx = 20/21... which when you plug into 2tanx/(1-tan^2(x)) Is 2(20/21) / (1-(20/21)^2) Which is.... (40/21) / (441/441 - 400/441) = (840/441) / (41/441) = 840/41 But that's postive... Wait..... tanx should be -20/29.... which makes it negative... okay... i get it :S But for number 8 she made an error right? __________________ Can I sail through the changing ocean tides, can I handle the seasons of my life?

16th Nov 2011, 12:28 AM	#7
Mad Man L EC Addict Full Member Gender: Male Orientation: Bisexual Out Status: All but family Location: Brisbane, Australia Age: 16 Posts: 1,244 Join Date: Jul 2011	Re: Trig help... please? :3 There's a typo in Question 8. 3(pi)/2 < A < (pi) Means: 3(pi)/2 < (pi) Which is obviously wrong. Assuming they meant this: (pi) < A < 3(pi)/2, you get the angle of 210. (Working in degrees here, radians look ugly). B is around 165.52 degrees (cba exact value), but all in all you get sin 44.48 = 0.7006. Subtract this from D, and it's pretty much correct. __________________ I really should get a proper signature...

16th Nov 2011, 01:52 AM	#8
Austin Full Member Gender: Male Orientation: Gay Out Status: Most people in my life. Location: Orange County, California, USA Age: 19 Posts: 1,790 Join Date: Apr 2008	Re: Trig help... please? :3 Oh! Thanks for pointing that out... for some reason I kept getting 3pi/2 as 135 ugg idk why hahaha. So it's 270? So it's in Quadrant 3..... i think i asked her this when we were taking the test, I said "Is this right...?" And she said yes O_o but guess she didn't get what I was saying lol. So apparently I was not thinking for either of these two problems. I guess I'll have to redo that... :P not sure what u mean by "Subtract this from D, and it's pretty much correct." Thanks so much for the help guys! __________________ Can I sail through the changing ocean tides, can I handle the seasons of my life?

16th Nov 2011, 02:48 AM	#9
Doctor Faustus EC's resident Philosopher at Large Regular Member Gender: Male Orientation: Not straight. But only interested in men. xD Out Status: People who ask me. People whom I trust. Location: Basingstoke Posts: 1,610 Join Date: Oct 2011	Re: Trig help... please? :3 Was about to reply, but glad it was sorted out! Need algebra and lit help? (points) __________________ "I have found the paradox, that if you love until it hurts, there can be no more hurt, only more love." - Mother Teresa.

16th Nov 2011, 03:29 AM	#10
caughtbywitness Well Known Full Member Gender: Male Orientation: I really don't know anymore Out Status: Most people (: Location: Shropshire Age: 16 Posts: 131 Join Date: Oct 2011	Re: Trig help... please? :3 Just took my maths exam. FML.

18th Nov 2011, 07:46 AM	#13
Ionless Depressed, Lost. Seeking Solace. Full Member Gender: Male Orientation: about a 4.5 Out Status: Family only Age: 19 Posts: 162 Join Date: Dec 2008	Re: Trig help... please? :3 Not sure if you learnt this but ... sin (A+B) = sinAcosB + cosAsinB sin (A-B) = sinAcosB - cosAsinB cos (A+B) = cosAcosB - sinAsinB cos (A-B) = cosAcosB + sinAsinB tan (A+B) = (tan A + tan B)/(1-tanAtanB) tan (A-B) = (tan A - tan B)/(1+tanAtanB) Exactly how these identities are derived I cannot remember anymore haha. But using sin(A-B) = sinAcosB - cosAsinB and using a right angled triangle and assuming that it is 3pi/2 > A > pi, sin(A-B) = (-1/2)(-sqrt 15 /4) - (-sqrt 3 /2)(1/4) = sqrt 15 /8 + sqrt 3 /8 = (sqrt 15 + sqrt 3) / 8 which is option D

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