# Multivariable calculus

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Question 1) Find the direction from (-3, 1, 2), in which g(x,y,z)=x^2*y*z-2*z^3 decreases fastest.

question 2) Follow the line in the direction you found in part 1) to estimate, using linear approximation, the location of the point closest to the coordinates (-3,1,2) at which g=1;Do not use a calculator; Express the answer using fractions. Next, use a calculator to evaluate g at your point.(the value should be reasonably close to 1).

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This solution shows how to find the direction from in which a function decreases fastest.

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1) Find the direction from P(-3, 1, 2), in which g(x,y,z)=x^2*y*z-2*z^3 decreases fastest.

Solution. Since , we have

........(1)

So,

So, we know that the gradient of at P(-3, 1, 2) is

We know that The direction of grad g(x,y,z) is the direction with maximal directional derivative. The direction of -grad ...

###### Education

- BSc , Wuhan Univ. China
- MA, Shandong Univ.

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- "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
- "excellent work"
- "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
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