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# C1 Homework helping watch

1. How would I find the equation for l. I have part a but I need b and c.
2. So you're finding the gradient of the curve at the origin, (0, 0)
You have a point, you have the gradient of the line, that's all you need to find the equation of the line .
3. (Original post by B_9710)
So you're finding the gradient of the curve at the origin, (0, 0)
You have a point, you have the gradient of the line, that's all you need to find the equation of the line .
How do I find the gradient with only one point?
4. If I remember correctly you differentiate twice then sub in the x co-ordiante to get the gradient of the curve at that point.
5. (Original post by TheStudent19)
If I remember correctly you differentiate twice then sub in the x co-ordiante to get the gradient of the curve at that point.
I know how to differentiate but this is only c1 chapters 1-5 so we havent done differentiation in core, just in FP1. Thats what confused me.
6. (Original post by Youngey4)
I know how to differentiate but this is only c1 chapters 1-5 so we havent done differentiation in core, just in FP1. Thats what confused me.
Hmm good point mate. What year paper is this? Is it edexcel?
7. (Original post by TheStudent19)
Hmm good point mate. What year paper is this? Is it edexcel?
Yeah its edexcel but i'm not sure what year as its just a booklet our teacher put together to do over half term
8. The only way I would go about doing b is to do c first. To do c you make the two equations equal and then find the co-ordinates I think.

EDIT: Just realised that you can't do that. Give me another minute Do you have the answers?
9. (Original post by Youngey4)
I know how to differentiate but this is only c1 chapters 1-5 so we havent done differentiation in core, just in FP1. Thats what confused me.
There is another way to do it without differentiation, although I do think that this question is designed so that you use differentiation.
You have and suppose we have a line through the origin . Then solving these simultaneously you get right.
Now we know that the line is tangent to the curve at the origin.So you can write in the form since you know that there is a repeated root at the origin - which is where the comes from.
So this is an identity, .
Should be straight forward to solve for a and c and so both parts of the question are done.
This way is similar to when you use the discriminant to find points where a line is tangent to a parabola.
In fact for this question you could use the discriminant of a cubic to get the answer but it's quite long and most people don't know it off the top of their heads I'd imagine.

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