# What is the equation of the line that is normal to #f(x)= x/sqrt( 3x+2) # at # x=4 #?

##### 1 Answer

Jan 9, 2016

Do this:

1)Find f'(x)

This is the slope of the tangent line to the graph of f(x). Let's call it m_t.

You want to find m_n, which is -1/(m_t). Recall if two lines are perpendicular, their respective slopes are negative reciprocals of one another. I've inserted a pretty TI screenshot, displaying the graph of f(x) with tangent line and normal line intersection.

What is that point of intersection?

2)Find the point (4,f(4)).

3)Use the point-slope equation for a line

y-y1=(m_n)(x-x1), where x1=4 and y1=f(4) and (m_n)=f'(4)

That's your equation for the line.

Your f'(x) should look like this....I don't want to do a step by step, cause that's your job...