# Math Myth: What is a tangent?

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From @fresh_42's Insight
https://www.physicsforums.com/insights/10-math-things-we-all-learnt-wrong-at-school/

Please discuss!

Yes, it is the derivative of ##y.## But what is meant by that? Obviously we have a function ##x \longmapsto y=y(x)## and a derivative $$y'=y'(x)=\dfrac{dy}{dx}=\left. \dfrac{d}{dx}\right|_{x=a}y(x)=y(a+h)-J(h)-r(h)=y'(a)$$ It now isn't obvious at all what is meant: the function ##x\longmapsto y'(x)##, the value of the slope ##y'(a)##, or the linear map ##J,## the Jacobi matrix, the tangent in a way? Fact is, all of them, as needed according to the situation. I don't say we should teach tangent bundles and sections, but a little bit more accuracy would smoothen the step to calculus at college.

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## Answers and Replies

Stephen Tashi
Science Advisor
In the USA, a calculus problem asking for the tangent of ##f(x)## at ##x = a## is understood to ask for the equation of a line tangent to the graph at ##f(x)## at ##x = a## so ##f'(x)|_{x=a}## is a wrong answer.