Sunday, September 14, 2014

Find the equation of the tangent to the curve y = 3x - x^3 at x = 2 .

To determine the equation of the tangent line to the graph
of y, we'll have to determine the derivative of y at x =
2.


f'(2) = lim [f(x) -
f(2)]/(x-2)


f(2) = 3*2 -
2^3


f(2) = 6 - 8


f(2) =
-2


lim [f(x) - f(2)]/(x-2) = lim (3x - x^3 +
2)/(x-2)


We'll substitute x by
2:


lim (3x - x^3 + 2)/(x-2) = (6-8+2)/(2-2) =
0/0


Since we've obtained in indeterminacy acse, we'll apply
L'Hospital rule:


lim (3x - x^3 + 2)/(x-2) = lim (3x - x^3 +
2)'/(x-2)'


lim (3x - x^3 + 2)'/(x-2)' = lim
(3-3x^2)/1


We'll substitute x by
2:


lim (3-3x^2)/1 = 3 - 3*4 = 3-12 =
-9


f'(2) = -9


But the
derivative of y at x = 2 is the slope of the tangent line to the curve
y.


m = -9


Now, we'll write the
equation of the tangent line, whose slope is m=-9 and it passes through the point that
has x coordinate = 2.We'll compute the y coordinate of this
point:


f(2) = 3x - x^3


f(2) =
6 - 8


f(2) = -2


The equation
of the tangent line is:


y - (-2) = m(x -
2)


y + 2 = -9(x - 2)


We'll
remove the brackets:


y + 2 = -9x +
18


y + 9x + 2 - 18 =
0


y + 9x - 16 =
0

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