Sunday

November 23, 2014

November 23, 2014

Posted by **Mandy** on Friday, November 2, 2007 at 7:52am.

Here's what I have:

Sqrt {(x-0)^2 + ((x^2)-1)}

Sqrt {((x^2) + (x^2)-1}

d= sqrt (x^4)-1

y= (x^4)-1

Fprime(x) = 4x^3

Domain = ARN

Critical Number x= 0

Any help or comments would help alot. Thanks!

- Calculus I -
**Reiny**, Friday, November 2, 2007 at 8:24amMandy, you messed up in your first line

you forgot to square the last term in the brackets

dist = √((x-0)^2 + (y-1)^2)

=√(x^2 + (x^2 - 1)^2)

=√(x^4 - x^2 + 1)

then d(dist)/dx = 1/2(x^4 - x^2 + 1)^(-1/2)(4x^3 - 2x)

if we set this to zero we get

4x^3 - 2x = 0

for x=±1/√2 sub back to get y = 1/2

so the point is (1/√2,1/2) in the first quadrant

there is a second point in the second quadrant which is the same distance away because of the symmetry of the parabola.

- Calculus I -
**Reiny**, Friday, November 2, 2007 at 8:36amthere is an easier way to do these kind of questions.

It uses the fact that at the "closest" point the tangent must be perpendicular to the line from the given point to that tangent

So let the point on the curve be (a,a^2)

form y=x^2

dy/dx = 2x, so at (a,a^2) the slope of the tangent is 2a

the slope of the line from (0,1) to the tangent is then (a^2 - 1)/a

from the perpendicular line slope property

(a^2 - 1)/a = -1/(2a)

to get a = ±1/√2 as above

(notice that we also get a=0, which is correct according to the condition we imposed on the equation, namely that the lines had to be perpendicular.)

- Calculus I -
- Calculus I -
**drwls**, Friday, November 2, 2007 at 8:32amYou started out doing it correctly, but made an algebraic error.

d^2 = x^2 + (x^2-1)^2

d is a minimum where d^2 is a minimum, so you don't have to deal with the square root in finding the minimum distance.

d/dx(d^2) = 2x + 2(x^2 -1)* 2x

= 2x +4x^3 -4x = 4x^3 -3x

Set that = 0 and solve for x

One solution is x=0, where y=0

Another solution is

Another solution is where

4x^2 - 3 = 1

x = sqrt (3/4) = +or- 0.866

y = 0.75

Not all of these three points represent minima. Try them out (compute d^2) or use the second derivative test to find the true minimum. I get the minimum d^2 to be at

x = sqrt (3/4) = +or- 0.866

where y = 0.75

- Calculus I -
**drwls**, Friday, November 2, 2007 at 8:35amI seem to disagree with some of the others above, so check themn all for accuracy.

**Answer this Question**

**Related Questions**

Calculus - Please look at my work below: Solve the initial-value problem. y'' + ...

Math(Roots) - sqrt(24) *I don't really get this stuff.Can somebody please help ...

Math/Calculus - Solve the initial-value problem. Am I using the wrong value for ...

Math Help please!! - Could someone show me how to solve these problems step by ...

math calculus please help! - l = lim as x approaches 0 of x/(the square root of...

Math - How do you find a square root of a number that's not a perfect square? I'...

Mathematics - sqrt 6 * sqrt 8 also sqrt 7 * sqrt 5 6.92820323 and 5.916079783 So...

Calculus - Evaluate the indefinite integral: 8x-x^2. I got this but I the ...

Inequality - When I solve the inquality 2x^2 - 6 < 0, I get x < + or - ...

Calculus - Graph the curve and find its exact length. x = e^t + e^-t, y = 5 - 2t...