I need to know the steps of how to do this because there is a similar question

Not sure if this is the steps/methodology you're meant to use but I can explain how I came to the solution. So the power rule dictates that the derivative of a(x^b) is equal to ab[x^(b-1)]. So if you reverse the power rule for 4x and -7 you end up with f(x)=2x^2-7x. Both of these terms contain variables, and you know that f(0) is equal to 8, so there must also be a constant of 8. This brings you to f(x)=2x^2-7x+8. Let me know if you need further explanation or I was unclear.

This one>?

Just take the integral of both sides, gives f(x) = 2x^2 - 7x = C

Then you are told f(0) = 8, so use that info to solve for C

solve the equation for y in terms of x

use this solution to substitute into the constraint, eliminating y in the constraint.

set the constraint equal to a new variable, say k; so k is a function of x.

calculate dk/dx

set dk/dx equal to 0 and solve for x.

plug this solution for x into the original equation to get your solution for y.

Find positive numbers x and y satifying the equation xy=15 such that the sum 3x+y is as small as possible?

Solve this quick please! whats the values of x and Y!

you also need to this for y.

remember that integral of 1/x = ln|x| so you can choose the negative value [for a smaller answer]

i get:

x = -15.337 = -e^(-1/45)
y = -0.9780 = 15/x

This post was edited by rx7drifter on Nov 4 2013 08:03pm

two problems with that...

• never use appoximations unless you are asked too
• the problem asked for positive solutions for x and y

also without the negative rule above, I can come up with a better solution by inspection: x=-30, y = -1/2
and better than that, x= -60, y = -1/4, and so on.

This post was edited by Azrad on Nov 4 2013 08:10pm

perhaps you took the integral of the constraint instead of the derivative? Might explain where those e's and natural logs came from...