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# C4 - Differentiating parametric equations watch

1. I was doing some C4 - Differentiating parametric equations homework from the edexcel textbook... I keep getting different answers when I plug in my numbers for example:

Differentiating x=3t-2sint y=t^2+tcost..
so:
dx/dt =3−2cost, dy/dt =2t+  −tsint+cost 
dy/dx 2t − t sin t + cos t / 3−2cost

I plug in pi/2 and keep getting 4.02... , the answer should be 0.52.. / pi/6

I did reset my calculator but I get the same answer.
Any idea why this happens?
2. (Original post by Rukia12)
I was doing some C4 - Differentiating parametric equations homework from the edexcel textbook... I keep getting different answers when I plug in my numbers for example:

Differentiating x=3t-2sint y=t^2+tcost..
so:
dx/dt =3−2cost, dy/dt =2t+  −tsint+cost 
dy/dx 2t − t sin t + cos t / 3−2cost

I plug in pi/2 and keep getting 4.02... , the answer should be 0.52.. / pi/6

I did reset my calculator but I get the same answer.
Any idea why this happens?

Your working is also quite unclear, particularly in the line dy/dx 'cost / 3 - 2cost' - I'm not sure what this would actually look like on paper or what you're referring to.

I would suggest using latex

$\frac{cost}{3 - 2cost}$ = for example.but I do not think that is what you mean as this is undefined at t= pi/2. So maybe you mean
3. (Original post by Rukia12)
I was doing some C4 - Differentiating parametric equations homework from the edexcel textbook... I keep getting different answers when I plug in my numbers for example:

Differentiating x=3t-2sint y=t^2+tcost..
so:
dx/dt =3−2cost, dy/dt =2t+  −tsint+cost 
dy/dx 2t − t sin t + cos t / 3−2cost

I plug in pi/2 and keep getting 4.02... , the answer should be 0.52.. / pi/6

I did reset my calculator but I get the same answer.
Any idea why this happens?
I got pi/6.

sin pi/2 = 1 cos pi/2 = 0

4. (Original post by SeanFM)

Your working is also quite unclear, particularly in the line dy/dx 'cost / 3 - 2cost' - I'm not sure what this would actually look like on paper or what you're referring to.

I would suggest using latex

$\frac{cost}{3 - 2cost}$ = for example.but I do not think that is what you mean as this is undefined at t= pi/2. So maybe you mean

this is my working out
5. (Original post by Muttley79)
I got pi/6.

sin pi/2 = 1 cos pi/2 = 0

No I'm in degrees, am I supposed to be in radians?
6. (Original post by Rukia12)
No I'm in degrees, am I supposed to be in radians?
Almost always you should be using radians (especially if you see pi...) unless the question specifies degrees.
7. (Original post by Rukia12)
No I'm in degrees, am I supposed to be in radians?
Yes - all trig in calculus must be in radians
8. (Original post by SeanFM)
Almost always you should be using radians (especially if you see pi...) unless the question specifies degrees.
In calculus d/dx(sinx) only equals cos x if x is in radians ...
9. (Original post by SeanFM)
Almost always you should be using radians (especially if you see pi...) unless the question specifies degrees.
(Original post by Muttley79)
In calculus d/dx(sinx) only equals cos x if x is in radians ...
Thank you very much I've got the right answer, I did try it in radians before but I think something was wrong with my set-up!
10. (Original post by Muttley79)
In calculus d/dx(sinx) only equals cos x if x is in radians ...
Sorry, that is what I was trying to say but worded poorly and no reference made to it . but that doesn't mattter now
11. (Original post by SeanFM)
Sorry, that is what I was trying to say but worded poorly and no reference made to it . but that doesn't mattter now
I knew you meant that but was just clarifying for the OP
12. (Original post by Rukia12)
Thank you very much I've got the right answer, I did try it in radians before but I think something was wrong with my set-up!
Well done for sorting it out - some of my students have two calculators and keep one in degrees and one in radians.

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