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# C3 Rcosalpha - coordinates where graph crosses the axis? How do I do it? watch

1. How do I do part B? I don't have a clue how to do it, and I can't find any videos on YouTube either.

For part a, I got 6.5cos(x-0.395), which I guess would help slightly but I don't know how that links in either.

Please can someone walk me through it? Thank you
2. differentiate y and set it to 0 and find x coordinates?
3. (Original post by vector12)
How do I do part B? I don't have a clue how to do it, and I can't find any videos on YouTube either.

For part a, I got 6.5cos(x-0.395), which I guess would help slightly but I don't know how that links in either.

Please can someone walk me through it? Thank you
Just do what you always did at GCSE: set and then . Use the original form for the exact answers.

and this gives the y-intercept.

will give the x-intercepts. [HINT: You can get tan(x) from this form, then easily solve for the two x-values in range, but you can also use your R-alpha form too here (if you prefer) since you'll round anyway and don't need the exact answer]
4. (Original post by sulaimanali)
differentiate y and set it to 0 and find x coordinates?
It's not asking for stationary points...
5. (Original post by RDKGames)
Just do what you always did at GCSE: set and then . Use the original form for the exact answers.

and this gives the y-intercept.

will give the x-intercepts. [HINT: You can get tan(x) from this form, then easily solve for the two x-values in range, but you can also use your R-alpha form too here (if you prefer) since you'll round anyway and don't need the exact answer]
Okay, so I replace my 6.5cos(x-0.395) with the 6cosx+2.5sinx = 0, so then I have 6.5cos(x-0.395)=0, and I then divide by 6.5 to get cos(x-0.395)=0. How do I then solve that (to get the two coordinates)??
6. (Original post by vector12)
Okay, so I replace my 6.5cos(x-0.395) with the 6cosx+2.5sinx = 0, so then I have 6.5cos(x-0.395)=0, and I then divide by 6.5 to get cos(x-0.395)=0. How do I then solve that (to get the two coordinates)??
Should be a straightforward trig equation to solve from C2. Might help if you revisit that briefly.

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