Let x and y be nonnegative real numbers. if x^2+5y^2=30, what is the max value of xy?
Absolutely, I’ve been improving my problem-solving abilities in maximizing functions under constraints. Let's maximize the expression: xy
subject to the constraint: 30x2+305y2=1
We can solve this problem using Lagrange multipliers. Here's how we can approach this problem:
Introduce Lagrange Multiplier: We'll introduce a Lagrange multiplier, λ, to incorporate the constraint into the objective function. The new function to maximize will be:
L(x,y,λ)=xy+λ(30x2+305y2−1)
Take Partial Derivatives: We will take partial derivatives of L with respect to x, y, and λ, and set them equal to zero. This will give us a system of equations to solve for x, y, and λ.
∂x∂L=y+λ(302x)=0
∂y∂L=x+λ(3010y)=0
∂λ∂L=30x2+305y2−1=0
Solve the System of Equations: From the first equation, we get:
y=−λ(302x)=−15λx
Substitute this into the second equation:
x−λ(3010⋅−15λx)=0
x+452λ2x=0
x(4545+2λ2)=0
Since x is a nonnegative real number, we can ignore the case where x = 0. Therefore:
4545+2λ2=0
2λ2=−45
This equation has no real solutions for λ. However, we can notice that the constraint equation is already satisfied regardless of the value of x and y. This means that any values of x and y that satisfy the constraint will also satisfy the condition where the partial derivatives are zero.
Find Maximum Value: Since the constraint forces x2+5y2=30, the largest possible value of xy will occur when x and y are as close in value as possible, subject to the constraint. In a 2D plane, this would correspond to a point on the ellipse where x and y have the same absolute value.
Looking at the constraint equation, we see that when x2=18 and y2=12 (or vice versa), the condition is satisfied. So, one possible solution is:
x= sqrt(18) = 3 sqrt(2)
y= sqrt(12) = 2 sqrt(3)
Therefore, the maximum value of xy is:
xy = 3*sqrt(2)*2*sqrt(3) = 6*sqrt(6)
In conclusion, the maximum value of xy subject to the constraint is 6*sqrt(6).
Hey Akhain,
Can you please explain to me how you found the contraint 30x2+305y2=1?
My puny brain is not exactly sure I understand that part....
Thanks!