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# AQA C1 Maths (Tangent Equations) watch

1. When it says you have to find the equation of the tangent from the curve, you are not required to use the negative reciprocal, but when the equation is a tangent from the circle you do need to use the negative reciprocal.

Can someone clarify to me as to why?
2. (Original post by iAstro)
When it says you have to find the equation of the tangent from the curve, you are not required to use the negative reciprocal, but when the equation is a tangent from the circle you do need to use the negative reciprocal.

Can someone clarify to me as to why?
When you find the gradient of the tangent from the curve, you need to use differentiation. If you try and do this using the equation of a circle it is much more difficult to differentiate. Therefore you find the gradient of the point to radius, as the tangent is perpendicular you take the negative reciprocal of this to give the gradient of your tangent. Once you have found the gradient you can substitute the coordinates in to give you the equation of the tangent.

At least this is what I understand of it (I'm in EdExcel so I may be completely missing your point-sorry)!

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3. (Original post by iAstro)
When it says you have to find the equation of the tangent from the curve, you are not required to use the negative reciprocal, but when the equation is a tangent from the circle you do need to use the negative reciprocal.

Can someone clarify to me as to why?
Remember the circle theorem that the angle between a radius and a tangent is 90 degrees?

That's how it is perpendicular.
4. Thanks ^ that makes much more sense to me now.

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