Use Lagrange multipliers to find the max and min of the function $f(x,y)=xe^y$ subject to the constraint...
$begingroup$
Use Lagrange multipliers to find the max and min of the function $f(x,y)=xe^y$ subject to the constraint $x^2+y^2=6$.
My attempt:
I tried to find both partials and set them equal to $lambda$ times the partial of the constraint and got the following equations
$$e^y=2x(lambda)$$
$$x(e^y)=2y(lambda) $$
I then solved for $x$ and $y$ and got :
$$x=frac{e^y}{2lambda}$$
$$y=frac{xe^y}{2lambda}$$
I then plugged those into the original equation and got
$$frac{(x^2+1)(e^(2y) )}{4lambda^2}=6$$
I'm confused where to go from here and any help will be greatly appreciated
multivariable-calculus lagrange-multiplier
$endgroup$
add a comment |
$begingroup$
Use Lagrange multipliers to find the max and min of the function $f(x,y)=xe^y$ subject to the constraint $x^2+y^2=6$.
My attempt:
I tried to find both partials and set them equal to $lambda$ times the partial of the constraint and got the following equations
$$e^y=2x(lambda)$$
$$x(e^y)=2y(lambda) $$
I then solved for $x$ and $y$ and got :
$$x=frac{e^y}{2lambda}$$
$$y=frac{xe^y}{2lambda}$$
I then plugged those into the original equation and got
$$frac{(x^2+1)(e^(2y) )}{4lambda^2}=6$$
I'm confused where to go from here and any help will be greatly appreciated
multivariable-calculus lagrange-multiplier
$endgroup$
$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10
add a comment |
$begingroup$
Use Lagrange multipliers to find the max and min of the function $f(x,y)=xe^y$ subject to the constraint $x^2+y^2=6$.
My attempt:
I tried to find both partials and set them equal to $lambda$ times the partial of the constraint and got the following equations
$$e^y=2x(lambda)$$
$$x(e^y)=2y(lambda) $$
I then solved for $x$ and $y$ and got :
$$x=frac{e^y}{2lambda}$$
$$y=frac{xe^y}{2lambda}$$
I then plugged those into the original equation and got
$$frac{(x^2+1)(e^(2y) )}{4lambda^2}=6$$
I'm confused where to go from here and any help will be greatly appreciated
multivariable-calculus lagrange-multiplier
$endgroup$
Use Lagrange multipliers to find the max and min of the function $f(x,y)=xe^y$ subject to the constraint $x^2+y^2=6$.
My attempt:
I tried to find both partials and set them equal to $lambda$ times the partial of the constraint and got the following equations
$$e^y=2x(lambda)$$
$$x(e^y)=2y(lambda) $$
I then solved for $x$ and $y$ and got :
$$x=frac{e^y}{2lambda}$$
$$y=frac{xe^y}{2lambda}$$
I then plugged those into the original equation and got
$$frac{(x^2+1)(e^(2y) )}{4lambda^2}=6$$
I'm confused where to go from here and any help will be greatly appreciated
multivariable-calculus lagrange-multiplier
multivariable-calculus lagrange-multiplier
edited Dec 2 '18 at 23:02
Emily
asked Dec 2 '18 at 22:56
EmilyEmily
133
133
$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10
add a comment |
$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10
$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
We have,
$$e^y = 2 lambda x$$
$$x e^y = 2 lambda y$$
thus,
$$x = frac{2 lambda y}{e^y} = frac{2 lambda y}{2 lambda x} = frac{y}{x}$$
finally,
$$x^2 = y$$
Now we replace into the original (constraint) equation
$$y^2 + y - 6 = 0$$
You have now the constraint in the form of a quadratic equation, you can easily find the value of $y$ through solving the equation.
$endgroup$
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
add a comment |
$begingroup$
Following your calculations:
$$2lambda=e^y/x$$
$$2lambda=xe^y/y$$
so
$$e^y/x=xe^y/y iff ye^y=x^2e^y iff y=x^2$$ where in the last equivalency we use that $e^yne0$.
Now, use this information with your constraint to get a bicuadratic equation on $x$ that you can solve alone.
$endgroup$
add a comment |
$begingroup$
Without Lagrange Multipliers
Calling
$$
x = rcos(theta)\
y = rsin(theta)
$$
we have the equivalent problem
$$
maxmin_{theta}f(theta) = sqrt{6}cos(theta) e^{sqrt 6sin(theta)}
$$
and
$$
f'(theta) = sqrt{6}e^{sqrt{6} sin (theta )} left(sqrt{6} cos ^2(theta )-sin (theta )right)to sqrt{6} cos ^2(theta )-sin (theta ) = 0
$$
etc.
NOTE
This result is equivalent to the system
$$
x^2+y^2=6\
x^2-y = 0
$$
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
We have,
$$e^y = 2 lambda x$$
$$x e^y = 2 lambda y$$
thus,
$$x = frac{2 lambda y}{e^y} = frac{2 lambda y}{2 lambda x} = frac{y}{x}$$
finally,
$$x^2 = y$$
Now we replace into the original (constraint) equation
$$y^2 + y - 6 = 0$$
You have now the constraint in the form of a quadratic equation, you can easily find the value of $y$ through solving the equation.
$endgroup$
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
add a comment |
$begingroup$
We have,
$$e^y = 2 lambda x$$
$$x e^y = 2 lambda y$$
thus,
$$x = frac{2 lambda y}{e^y} = frac{2 lambda y}{2 lambda x} = frac{y}{x}$$
finally,
$$x^2 = y$$
Now we replace into the original (constraint) equation
$$y^2 + y - 6 = 0$$
You have now the constraint in the form of a quadratic equation, you can easily find the value of $y$ through solving the equation.
$endgroup$
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
add a comment |
$begingroup$
We have,
$$e^y = 2 lambda x$$
$$x e^y = 2 lambda y$$
thus,
$$x = frac{2 lambda y}{e^y} = frac{2 lambda y}{2 lambda x} = frac{y}{x}$$
finally,
$$x^2 = y$$
Now we replace into the original (constraint) equation
$$y^2 + y - 6 = 0$$
You have now the constraint in the form of a quadratic equation, you can easily find the value of $y$ through solving the equation.
$endgroup$
We have,
$$e^y = 2 lambda x$$
$$x e^y = 2 lambda y$$
thus,
$$x = frac{2 lambda y}{e^y} = frac{2 lambda y}{2 lambda x} = frac{y}{x}$$
finally,
$$x^2 = y$$
Now we replace into the original (constraint) equation
$$y^2 + y - 6 = 0$$
You have now the constraint in the form of a quadratic equation, you can easily find the value of $y$ through solving the equation.
answered Dec 2 '18 at 23:22
Blg KhalilBlg Khalil
283
283
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
add a comment |
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
Thank you so much I just found the roots to be 2 and -3 but I'm confused what to do with those?
$endgroup$
– Emily
Dec 2 '18 at 23:43
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
$begingroup$
You repalce the values of $y$ that you have found in the constraint equation & you end up getting two points $(x_1,y_1)$ & $(x_2,y_2)$, one of these two points is the local max. while the other is the local min., to verify which one is the local max. & which one is the local min. you calculate the second order partial derivatives of $f(x,y)$ because it's a multivariate function you will calculate the Hessian matrix & its determinant to find out which of the two points is local min. & which is the local max.,more here: analyzemath.com/calculus/multivariable/maxima_minima.html
$endgroup$
– Blg Khalil
Dec 3 '18 at 2:28
add a comment |
$begingroup$
Following your calculations:
$$2lambda=e^y/x$$
$$2lambda=xe^y/y$$
so
$$e^y/x=xe^y/y iff ye^y=x^2e^y iff y=x^2$$ where in the last equivalency we use that $e^yne0$.
Now, use this information with your constraint to get a bicuadratic equation on $x$ that you can solve alone.
$endgroup$
add a comment |
$begingroup$
Following your calculations:
$$2lambda=e^y/x$$
$$2lambda=xe^y/y$$
so
$$e^y/x=xe^y/y iff ye^y=x^2e^y iff y=x^2$$ where in the last equivalency we use that $e^yne0$.
Now, use this information with your constraint to get a bicuadratic equation on $x$ that you can solve alone.
$endgroup$
add a comment |
$begingroup$
Following your calculations:
$$2lambda=e^y/x$$
$$2lambda=xe^y/y$$
so
$$e^y/x=xe^y/y iff ye^y=x^2e^y iff y=x^2$$ where in the last equivalency we use that $e^yne0$.
Now, use this information with your constraint to get a bicuadratic equation on $x$ that you can solve alone.
$endgroup$
Following your calculations:
$$2lambda=e^y/x$$
$$2lambda=xe^y/y$$
so
$$e^y/x=xe^y/y iff ye^y=x^2e^y iff y=x^2$$ where in the last equivalency we use that $e^yne0$.
Now, use this information with your constraint to get a bicuadratic equation on $x$ that you can solve alone.
answered Dec 2 '18 at 23:19
Tito EliatronTito Eliatron
1,446622
1,446622
add a comment |
add a comment |
$begingroup$
Without Lagrange Multipliers
Calling
$$
x = rcos(theta)\
y = rsin(theta)
$$
we have the equivalent problem
$$
maxmin_{theta}f(theta) = sqrt{6}cos(theta) e^{sqrt 6sin(theta)}
$$
and
$$
f'(theta) = sqrt{6}e^{sqrt{6} sin (theta )} left(sqrt{6} cos ^2(theta )-sin (theta )right)to sqrt{6} cos ^2(theta )-sin (theta ) = 0
$$
etc.
NOTE
This result is equivalent to the system
$$
x^2+y^2=6\
x^2-y = 0
$$
$endgroup$
add a comment |
$begingroup$
Without Lagrange Multipliers
Calling
$$
x = rcos(theta)\
y = rsin(theta)
$$
we have the equivalent problem
$$
maxmin_{theta}f(theta) = sqrt{6}cos(theta) e^{sqrt 6sin(theta)}
$$
and
$$
f'(theta) = sqrt{6}e^{sqrt{6} sin (theta )} left(sqrt{6} cos ^2(theta )-sin (theta )right)to sqrt{6} cos ^2(theta )-sin (theta ) = 0
$$
etc.
NOTE
This result is equivalent to the system
$$
x^2+y^2=6\
x^2-y = 0
$$
$endgroup$
add a comment |
$begingroup$
Without Lagrange Multipliers
Calling
$$
x = rcos(theta)\
y = rsin(theta)
$$
we have the equivalent problem
$$
maxmin_{theta}f(theta) = sqrt{6}cos(theta) e^{sqrt 6sin(theta)}
$$
and
$$
f'(theta) = sqrt{6}e^{sqrt{6} sin (theta )} left(sqrt{6} cos ^2(theta )-sin (theta )right)to sqrt{6} cos ^2(theta )-sin (theta ) = 0
$$
etc.
NOTE
This result is equivalent to the system
$$
x^2+y^2=6\
x^2-y = 0
$$
$endgroup$
Without Lagrange Multipliers
Calling
$$
x = rcos(theta)\
y = rsin(theta)
$$
we have the equivalent problem
$$
maxmin_{theta}f(theta) = sqrt{6}cos(theta) e^{sqrt 6sin(theta)}
$$
and
$$
f'(theta) = sqrt{6}e^{sqrt{6} sin (theta )} left(sqrt{6} cos ^2(theta )-sin (theta )right)to sqrt{6} cos ^2(theta )-sin (theta ) = 0
$$
etc.
NOTE
This result is equivalent to the system
$$
x^2+y^2=6\
x^2-y = 0
$$
answered Dec 3 '18 at 0:02
CesareoCesareo
8,6893516
8,6893516
add a comment |
add a comment |
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$begingroup$
Try to eliminate $lambda$ from the two equations.
$endgroup$
– Tito Eliatron
Dec 2 '18 at 23:08
$begingroup$
How would I go about doing that?
$endgroup$
– Emily
Dec 2 '18 at 23:10