Rathin's Maths Help Thread (1 Viewer)

[Green Yoda]

This thread is for my maths endeavours in my senior years of mathematical studies. I can be quite stupid at maths sometimes so bos is a great place for me to get help. Instead of making a 1000's of threads, I thought why not make 1 thread and ask for help and the math geniuses out there can help me out

To start of:
If 90°<θ<180°, use a unit circle to show that:
a) cos(180°+θ)=-cosθ
b) sin(360°-θ)=-sinθ

 

[kawaiipotato]

 

[Green Yoda]

I'm still not completely getting it.

 

[eyeseeyou]

lol what's with ppl creating maths threads? Lol

 

[leehuan]

Use this diagram just to visualise the part where kawaiipotato said "Understand that cos(theta) can be found by drawing a line...)

lol what's with ppl creating maths threads? Lol

I think everyone took my idea which I first got off Flop

 

[Green Yoda]

I understand that I draw a line in the 2nd quadrant. Which would mean that cos is - as the distance is - in terms of x distance and sin is + as he y distance is positive. But for cos(180°+θ) I dont understand what to do here.

 

[leehuan]

180 degrees is a straight angle

Draw the ray into the opposite quadrant

(Tbh - This question was dodgy in it's wording)

 

[Nailgun]

I understand that I draw a line in the 2nd quadrant. Which would mean that cos is - as the distance is - in terms of x distance and sin is + as he y distance is positive. But for cos(180°+θ) I dont understand what to do here.

rotating the triangle by 180 means that its in the third quadrant
the base is on the negative end of the x-axis
looking the point the x value is now -cosθ

hence cos(180+θ) = -cosθ

hence cos

 

[leehuan]

rotating the triangle by 180 means that its mirrored across the y axis
looking the point the x value is now -cosθ

hence cos(180+θ) = -cosθ

hence cos

Wait what?

 

[Nailgun]

Wait what?

lele i just edited it while thinking about it
i was moving the base around in my head
didnt realise the triangle was upside down

 

[Green Yoda]

Isnt theta in the 2nd quadrant first? Then when you rotate i 180°..it would be in the 4th quadrant?

 

[eyeseeyou]

I know a way of doing this but it's not the 4 quadrants methods, it's the unit circle method

Don't have my maths tutor book with me right now so will help you later

 

[leehuan]

Isnt theta in the 2nd quadrant first? Then when you rotate i 180°..it would be in the 4th quadrant?

If the second quadrant is negative, and the 4th quadrant is positive

The 4th quadrant is the negative of the second quadrant.

 

[Green Yoda]

If the second quadrant is negative, and the 4th quadrant is positive

The 4th quadrant is the negative of the second quadrant.

huh?

 

[Nailgun]

Isnt theta in the 2nd quadrant first? Then when you rotate i 180°..it would be in the 4th quadrant?

...isn't it in the first lol

 

[Green Yoda]

...isn't it in the first lol

If 90°<θ<180°..?

 

[Nailgun]

If 90°<θ<180°..?

oh lol didn't read the original question lel

 

[leehuan]

huh?

2nd quadrant - negative
4th quadrant - positive

What do you get when you take the negative of a negative? A positive. As required.

 

[Trebla]

This thread will be closed shortly.

After careful consideration, it was decided that user specific threads will no longer be allowed (whether they be one user helping or one user asking for help). We have noticed that the first few threads have spawned multiple others, which is not desirable in the spirit of being an open community. The last thing we want is the Maths forums being full of "User X Maths Help" threads.

You are encouraged to post separate questions in separate threads, though keep in mind a lot of the questions that I have seen so far have been answered before so I suggest to actually do a search first to see if it hasn't been answered already in past threads (which is easier to do when there is a specific thread for a specific question or topic rather than a general user specific thread).