For #f(x)=x^2/sqrt(x^3-5)# what is the equation of the tangent line at #x=5#?

1 Answer
Nov 9, 2015

#y=frac{7sqrt(30)}{192}x+frac{15sqrt(30)}{64}#

Explanation:

#f(5)=frac{5sqrt(30)}{12}#

#f'(x)=frac{d}{dx}[x^2(x^3-5)^{-1/2}]#

#=x^2frac{d}{dx}[(x^3-5)^{-1/2}]+(x^3-5)^{-1/2}frac{d}{dx}(x^2)#

#=x^2[(-1/2)(x^3-5)^{-3/2}(3x^2)]+(x^3-5)^{-1/2}(2x)#

#=frac{x(x^3-20)}{2(x^3-5)^{3/2}}#

#f'(5)=frac{7sqrt(30)}{192}#

Let the equation of the tangent line be

#y(x)=mx+c#,

where #m# and #c# are constants to be determined.

For a line to be tangent to a curve, they must intercept at the point of interest.

#y(5)=f(5)#

#5m+c=frac{5sqrt(30)}{12}#

In addition, the line and the curve should have the same gradient at the point of interception.

#y'(5)=f'(5)#

#m=frac{7sqrt(30)}{192}#

Solving for #c#,

#c=frac{5sqrt(30)}{12}-5frac{7sqrt(30)}{192}#

#=frac{15sqrt(30)}{64}#

#y(x)=frac{7sqrt(30)}{192}x+frac{15sqrt(30)}{64}#