Problem with NMaximize output [closed]

Question

This is what I'm trying to do:

sol = DSolve[{y''[x] - 4*y'[x] + 3*y[x] == 0, y[0] == 7, y'[0] == 11}, y[x], x]
Y[x] = y[x] /. sol
z = x /. Last[NMaximize[-x^2 - 4 x + 4, x]]
Y[z]

I want to find Y[z] but I get this output instead:

(* 
{{y[x] -> E^x (5 + 2 E^(2 x))}}
{E^x (5 + 2 E^(2 x))}
-2.
Y[-2.] 
*)

What should I do?

I'm sorry, but I had to downvote since this was already covered in the answer to your last question. You may not be aware of it, but there's a "virtual book" provided with Mathematica that gives a very good introduction to the language and should help clear up issues like this. If you don't know where to start with the documentation, it can be very helpful! You can get to it via the book icon in the Documentation Center, or via the Help menu.

belisarius · Accepted Answer · 2012-07-24 18:22:23Z

Replace your def for Y:

Y[x_] = y[x] /. sol

So it matches any argument value instead of just x

Edit

x_ is a pattern. Read about patterns in the help! It is of utmost importance!

Also, try this:

sol = DSolve[{y''[x] - 4*y'[x] + 3*y[x] == 0, y[0] == 7, y'[0] == 11}, y[x], x]
y[x] /. sol /. Last[NMaximize[-x^2 - 4 x + 4, x]]

closed as too localized by acl, rm -rf♦, Oleksandr R., Leonid Shifrin, Eli Lansey Jul 24 '12 at 18:54