Parallel Normal Lines between y = sqrt(x - 1) and y = 1 - 2x

In summary, the student is trying to find the normal line to the curve and solve for x. They found that x equals 2 when the derivative of sqrt(x-1) is -2.
  • #1
synergix
178
0

Homework Statement



Find the points where the normal line to the curve
y = sqrt(x - 1)
is parallel to the line y = 1 - 2x.

The Attempt at a Solution



m=-2

y' = 1/2sqrt(x-1) = m = -2

-4sqrt(x-1)=1

sqrt(x-1) = - 1/4

x= 1/16 + 1

x= 17/16
so find y
y = sqrt((17/16)-1))

(17/16, 1/4)
this is what I think I have to do but I know I am wrong just not sure why my instructor did not explain this question very well
 
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  • #2
You've made a credible attempt, if not correct, if the problem had been find the tangent line to the curve parallel to the line 1-2x. But it doesn't say that. It's asking you where the slope of the NORMAL line is -2. What's the relation between the slope of the tangent line and the normal line?
 
  • #3
the relationship between the the slope of the tangent line and the slope of the normal line is that the tangent will be the inverse negative of the slope of the normal line.

so slope of the normal line= -1(-2-1)

1/2sqrt(x-1) = m = 1/2

sqrt(x-1) = 1

x = 2
 
  • #4
Thanks! shouldn't be any trouble now.
 
  • #5
Wow, you've cleverly turned this completely upside down. Are you trying to confuse me? No, the slope of the normal line is supposed to be -2. It's supposed to be parallel to 1-2x. That's -2. What's the slope of the normal line to y=sqrt(x-1)?? Equate them.
 
  • #6
synergix said:
the relationship between the the slope of the tangent line and the slope of the normal line is that the tangent will be the inverse negative of the slope of the normal line.

so slope of the normal line= -1(-2-1)

1/2sqrt(x-1) = m = 1/2

sqrt(x-1) = 1

x = 2

What you did is find the normal slope to the line and equate it to the tangent direction of the curve. That's sort of backwards, but if you understand why that works, it's ok with me.
 
  • #7
Haha, damn now I am confused. The slope of the normal line to y=sqrt(x-1) is -2 at the points that I must find. So I need to know what does x equal when the derivative of [sqrt(x-1)]' = -2 that makes sense to me but it must be wrong because that's what I already did.
 
  • #8
Dick said:
What you did is find the normal slope to the line and equate it to the tangent direction of the curve. That's sort of backwards, but if you understand why that works, it's ok with me.

Ok would it work? I just learned this stuff yesterday and my teacher didn't really explain it very well. So maybe you have a more straight forward way.
 
  • #9
The slope of the tangent line is 1/(2sqrt(x-1)). So the slope of the normal line is -2sqrt(x-1). Same reasoning you used for the line. You want that to be -2. So you want -2=(-2)sqrt(x-1). It's the same equation you just solved. What you did is perfectly ok. Just want to make sure you understand why.
 
  • #10
OH, OK I get it now. thank you.
 

What is a point of normal line?

A point of normal line is a point on a curve or surface where the tangent line is perpendicular to the normal line.

How do you find the point of normal line?

To find the point of normal line, you need to first find the slope of the tangent line at that point. Then, take the negative reciprocal of that slope to find the slope of the normal line. Finally, use the point-slope formula to find the equation of the normal line passing through the given point.

What is the significance of a point of normal line in science?

Points of normal line have many applications in science, especially in fields such as physics and engineering. They are used to determine the direction of motion or force acting on an object, and also to find the rate of change of a function at a specific point.

Can a curve have multiple points of normal line?

Yes, a curve can have multiple points of normal line. This happens when the curve changes direction or curvature at different points, causing the tangent line to be perpendicular to the curve at those points.

How are points of normal line related to points of inflection?

Points of normal line and points of inflection are related in that they both occur at points where the curvature of a curve changes. However, a point of normal line is where the tangent line is perpendicular to the curve, while a point of inflection is where the concavity of the curve changes from concave up to concave down or vice versa.

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