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# differentiation watch

1. Hi,
I was doing the question:
Find the equation of the normal to the curve with parametric equations x=e^t y=e^t +e^-t where t=0

so i had no problem finding dy/dx by the chain rule and then getting the gradient of the tangent to be 0.

the first problem was not being able to use-1/m where m is the gradient to find the gradient of the normal and just having to recognise that its 1... so is it true that the rule goes out the window in this case.

Secondly, when finding the equation is it true you cant use y-y1=m(x-x1) in this case (only if you were asked to find the equation of the tangent and the gradient is 0) and instead find the x co-ordinate at t=0 as you know it is going to be of the form x=t. (where t in this case is 1)

Is this the case or am i missing something?
Thanks
2. (Original post by 111davey1)
Hi,
I was doing the question:
Find the equation of the normal to the curve with parametric equations x=e^t y=e^t +e^-t where t=0

so i had no problem finding dy/dx by the chain rule and then getting the gradient of the tangent to be 0.

the first problem was not being able to use-1/m where m is the gradient to find the gradient of the normal and just having to recognise that its 1... so is it true that the rule goes out the window in this case.

Secondly, when finding the equation is it true you cant use y-y1=m(x-x1) in this case (only if you were asked to find the equation of the tangent and the gradient is 0) and instead find the x co-ordinate at t=0 as you know it is going to be of the form x=t. (where t in this case is 1)

Is this the case or am i missing something?
Thanks
You cannot use the negative reciprocal if your gradient is 0 - a slightly different way of thinking is required.

At you have the point .

Since the gradient at that point is 0 then that means the tangent is of the form (which you can find quite simply if you apply ) so the normal must be of the form where are constants and the point where the two lines intersect gives . So what must the normal be if and are not dependent on each other??

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