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anonymous
 one year ago
For supplemental problem 2, G2 :
http://ocw.mit.edu/courses/mathematics/1802scmultivariablecalculusfall2010/2.partialderivatives/partafunctionsoftwovariablestangentapproximationandoptimization/problemset4/MIT18_02SC_SupProb2.pdf
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Isn't it enough to simply show the equation always has solutions as long as the determinant is not zero? And that if all the xi's are the same, the determinant will be zero, and thus no solution (in a vertical line)?
In other words, what does the Hint mean? How do we apply the Hint to solve the problem?
anonymous
 one year ago
For supplemental problem 2, G2 : http://ocw.mit.edu/courses/mathematics/1802scmultivariablecalculusfall2010/2.partialderivatives/partafunctionsoftwovariablestangentapproximationandoptimization/problemset4/MIT18_02SC_SupProb2.pdf , Isn't it enough to simply show the equation always has solutions as long as the determinant is not zero? And that if all the xi's are the same, the determinant will be zero, and thus no solution (in a vertical line)? In other words, what does the Hint mean? How do we apply the Hint to solve the problem?

This Question is Closed

phi
 one year ago
Best ResponseYou've already chosen the best response.0The "equation (4)" they refer to in the question is from http://ocw.mit.edu/courses/mathematics/1802multivariablecalculusfall2007/readings/least_sq_intrpol.pdf which in turn is found at http://ocw.mit.edu/courses/mathematics/1802multivariablecalculusfall2007/readings/supp_notes/

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Equation 4 will always have a solution as long as the deternimnant of \[\left[\begin{matrix}s ^{} & x^{} \\ x^{} & 1\end{matrix}\right]\] is not equal to zero right? So what does the hint: "Hint: use the fact that for all values of u we have \[\sum_{i=1}^{n}(x _{i}u)^{2}\ge0\] since squares are always nonnegative. Write the left side as a quadratic polynomial in u. Usually it has no roots. What does this imply about the coefficients? When does it have a root? (answer these two questions by using the quadratic formula)" Isn't all that unnecessary?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0*so what does the hint mean? Isn't all that unnecessary?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Ok so here's my reasoning Since for \[f(u) = \sum_{i=1}^{n}(x _{i}u)^{2}= n u ^{2}+u(2\sum_{i=1}^{n}x _{i}) + \sum_{i=1}^{n}x _{i}^{2}\ge0\], f(u) will have at most one root when the discriminant is equal to zero. This means that the discriminant is always less than or equal to 0, i.e. \[(2\sum_{i=1}^{n}x _{i})^{2}4n \sum_{i=1}^{n}x _{i}^{2}\le0\] i.e. usually, \[n \sum_{i=1}^{n}x _{i}^{2} (\sum_{i=1}^{n}x _{i})^{2}>0\]This is the determinant for equation 4. The fact that this is usually >0 means that in most cases there will be a solution. The solution will only not exist when \[n \sum_{i=1}^{n}x _{i}^{2}= (\sum_{i=1}^{n}x _{i})^{2}\], i.e. when all xi are the same. i.e. it is a vertical line Is this reasoning correct?

phi
 one year ago
Best ResponseYou've already chosen the best response.0I assume by the "determinant of equation 4" you mean after you put the equations in matrix form? But yes, your argument looks good.
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