Got a question with substitution?

[MRAI]

Using the Substitution method, find the maximum value of

4x-2xy+3y subject to the constraint 4x-y=2

I can do the 1st part:

4x-y=2--->
4x-2=y

substitute this into the original equation:
4x-2x(4x-2)+3(4x-2)

Hope that right so far! but don't know where to go from there? anyone could help please?

thanks ;)

 

[CartoonKid]

then you differentiate wrt x and equal to zero to find max/min

 

[MRAI]

what do u mean by wrt?

 

[HallsofIvy]

"wrt" : "with respect to"
CartoonKid was assuming this was a calculus problem. The standard method for solving "maximum- minimum" problems in calculus is to differentiate and set the derivative equal to 0. But since you apparently weren't aware of that, I suspect this is not a calculus problem!

If you multiply y= 4x-2x(4x-2)+3(4x-2) out, you get a quadratic. If you are doing problems like this, I presume you know how to complete the square to find the vertex of the graph which, in this problem, gives you the maximum value.

So: multiply this out, then complete the square.

 

[dextercioby]

This method is totally counterproductive...The method discovered by Lagrange is much more elegant...

Daniel.

 

[Theelectricchild]

But one would need a rather intermediate mathematical background to use them...