- #1
Yosty22
- 185
- 4
Homework Statement
Find the shortest distance from the origin to the surface x=yz+10
Homework Equations
The Attempt at a Solution
So I said that my main function, f(x,y,z) = x^2 + y^2 + z^2 (the function I want to minimize)
Then I said that g(x,y,z) is my constraint function where g(x,y,z) is yz-x=-10. I took the partial derivative with respect to each variable of both g and f. I got fx=2x, fy=2y, fz=2z, gx=-1, gy=z, and gz=y. Once I did this, set fx = λ gx etc. (same format for each partial). This is where I am confused.
My final equations are:
2x + λ = 0 (1)
2y - λz = 0 (2)
2z - λy = 0 (3)
yz - x = 10 (4)
Once I have these, I am confused as to how to solve them properly. What I did so far was solve equation 2 for z. Once I solved for z in terms of y and λ, I substituted it back into equation 3 and got 4y/λ - λy = 0. Multiplying lambda across, I get 4y = λ2y. This shows me that either λ=2 or y=0. Once I get these, for each case I solved and when y = 0, plugging back into equation 2, I get z = 0, and this means that x=10 (equation 4). However, if λ = 2, then by equation 1, x=-1.
My question is:
What should I be looking for here? What do I solve for to answer the question properly?