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C4 - Differentiating parametric equations watch

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    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

    Finding the gradient when t=pi/2
    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?
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    (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

    Finding the gradient when t=pi/2
    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?
    Are you using radians on your calculator?

    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

    [latex]\frac{cost}{3 - 2cost}[/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
    \frac{cost}{3} - 2cost
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    (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

    Finding the gradient when t=pi/2
    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

    Check you are in radians.
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    (Original post by SeanFM)
    Are you using radians on your calculator?

    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

    [latex]\frac{cost}{3 - 2cost}[/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
    \frac{cost}{3} - 2cost
    Name:  Screen Shot 2016-11-05 at 11.22.55.png
Views: 58
Size:  27.3 KB
    this is my working out
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    (Original post by Muttley79)
    I got pi/6.

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

    Check you are in radians.
    No I'm in degrees, am I supposed to be in radians?
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    (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.
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    (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
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    (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 ...
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    (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!
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    (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 :rofl:. but that doesn't mattter now
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    (Original post by SeanFM)
    Sorry, that is what I was trying to say but worded poorly and no reference made to it :rofl:. but that doesn't mattter now
    I knew you meant that but was just clarifying for the OP
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    (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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