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C4 Cartesian Parametric Question - why can they suddenly use this? watch

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    I get part A. It's part B which I don't get. Usually with cartesians, you get one equation in terms of the parametric (eg if you had x=2t and y=4t, you could do t=x/2 and sub that into the second equation), whereas with this one, they haven't done that.

    They have just used the fact that cos2t = 1-2sin^2t and used that. How do you know to do that? I could never have spotted it.

    Thanks
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    (Original post by stuart4)
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    I get part A. It's part B which I don't get. Usually with cartesians, you get one equation in terms of the parametric (eg if you had x=2t and y=4t, you could do t=x/2 and sub that into the second equation), whereas with this one, they haven't done that.

    They have just used the fact that cos2t = 1-2sin^2t and used that. How do you know to do that? I could never have spotted it.

    Thanks
    Just practise. If you've got sines and cosines, then you look for a way to eliminate them either via an identity, like cos^2+sin^2=1, or in this case since since one is cos2x and the other sin x, it screams double angle formula, and look for a way to use it.
 
 
 
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