Question

If
f(x) = 6x2 − x3,
find f '(1) and use it to find an equation of the tangent line to the curve
y = 6x2 − x3
at the point (1, 5).

Answer 1

Evaluate the derivative of f(x) at x = 1. I'll use the limit definition:

$\displaystyle \lim_{x\to1} \frac{f(x)-f(1)}{x-1} = \lim_{x\to1} \frac{6x^2-x^3-5}{x-1} = \lim_{x\to1}(-x^2+5x+5) = 9$

Then using the point-slope formula, the equation of the tangent line to f(x) at the point (1, 5) is

y - 5 = 9 (x - 1)

y = 9x - 4