What is the equation of the tangent line of #f(x)=(x-5)/(x+1) # at #x=6#?

1 Answer
Jun 13, 2016

First, we must differentiate #f(x)# using the quotient rule.

Explanation:

Let #f(x) = (g(x))/(h(x))#, then #f'(x) = ((g'(x) xx h(x)) - (h'(x) xx g(x)))/(h(x))^2#

Differentiating #g(x)# using the power rule:

#g'(x) = 1#

Differentiating #h(x)# using the power rule:

#h'(x) = 1#

Substituting:

#f'(x) = ((g'(x) xx h(x)) - (h'(x) xx g(x)))/(h(x))^2#

#f'(x) = ((1(x + 1)) - (1(x - 5)))/(x + 1)^2#

#f'(x) = (x + 1 - x + 5)/(x + 1)^2#

#f'(x) = (6)/(x + 1)^2#

Now, we have to plug the given #x# value into the derivative to find the slope of the tangent at that given point.

#f'(6) = 6/(6 + 1)^2#

#f'(6) = 6/49#

Now that we know the slope, let's find the value of y in #(6, y)# by plugging in #x = 6# into the original function:

#f(6) = (6 - 5)/(6 + 1)#

#f(6) = 1/7#

We currently know a point #(6, 1/7)# and the slope of the tangent, which is #6/49#.

We can now use point slope form to determine the equation of the tangent.

#y - y_1 = m(x - x_1)#

#y - 1/7 = 6/49(x - 6)#

#y - 1/7 = 6/49x - 36/49#

#y = 6/49x - 36/49 + 1/7#

#y = 6/49x - 29/49#

The equation of the tangent is #y = 6/49x - 29/49#.

Hopefully this helps!