What is the equation of the line that is normal to f(x)= sqrt( 3x+2) at x=1 ?

1 Answer
Jun 5, 2016

color(blue)(y=-sqrt(5)/3 x + (4sqrt(5))/3color(white)(2/2)

Explanation:

Write as y=(3x+2)^(1/2)

Using calculus

Let u=3x+2 => (du)/(dx) = 3....(1)

By substitution

y=u^(1/2) => (dy)/(du)=1/2u^(-1/2) ....(2)

By comparing equations (1) and (2)

(dy)/(dx)=(dy)/(cancel(du))xxcancel(du)/(dx)

=>(dy)/(dx)=1/2u^(-1/2)xx3

=>(dy)/(dx)=3/(2sqrt(3x+2))

Multiply by 1 but in the form of 1=sqrt(3x+2)/sqrt(3x+2)

dy/(dx) =(3sqrt(3x+2))/(3x+2)
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
color(blue)("Determine gradient of normal")
Condition for x=1

Gradient of given curve -> (3sqrt(5))/5

"So gradient of normal is "-> -5/(3sqrt(5))

Multiply by 1 but in the form of 1=(sqrt(5))/(sqrt(5)) giving:

" "-(cancel(5)sqrt(5))/(3xxcancel(5))

color(brown)("gradient "->-(sqrt(5))/3
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
color(blue)("Determine point through which the normal passes")

For x=1

y=sqrt(3x+2) " "vec("becomes")" " y=sqrt(5)

=> "Point "->(x,y)->(1,sqrt(5))

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
color(blue)("Determine equation of normal")

y=-1/mx+c" "vec("becomes")" " sqrt(5)=(-sqrt(5)/3xx1)+c

add" "sqrt(5)/3 to both sides

sqrt(5)+sqrt(5)/3=0+c

=> c= (4sqrt(5))/3

color(blue)(y=-sqrt(5)/3 x + (4sqrt(5))/3color(white)(2/2)