Tangent to a a curve, something seems wrong (Calculus)

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SUMMARY

The discussion clarifies the definition of the tangent line to a curve at a point, specifically using the formula y = f'(a)(x-a) + b. It emphasizes that f'(a) represents the slope at a single point, while (y-b)/(x-a) calculates the slope between two points on the tangent line, not the curve itself. The participants confirm that while this formulation is correct, it is essential to understand that x and y must correspond to points on the tangent line. The conversation also touches on the implications of this relationship for L'Hôpital's rule.

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christian0710
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I'm studying Calculus and i can see that the definition of the tangent to a point on a curve is

y= f'(a)(x-a)+b this must mean that

f'(a) = (y-b)/(x-a)

But that to me seems troubeling, because f'(a) is the slope at ONE point, while (y-b)/(x-a) is a quotient with the difference between 2 points. Is there a better explanation?
 
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No, that's perfectly correct. However, "f'(a)= (y- b)/(x- a)" requires that x and y be the coordinates of a point on the tangent line, not the curve. A line has the same slope at any point.
 
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Comparing this form to y = mx+k, you have f'(a) = m giving you the slope of the tangent at x=a. Your y- intercept of the line is b-f'(a) a, this is assuming that f(a) = b. Essentially, you are tracing back the line along the slope to find the y-intercept.

(y-b)/(x-a) is 0/0 at (a,b), so might not be the best form to use...although it does give some insight to L'Hopital's rule.
 
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