Message Boards Message Boards

GROUPS:

[✓]  One simple equation gets answer, similar one does not?

Posted 1 year ago
2132 Views
|
8 Replies
|
2 Total Likes
|

In Mathematica 11.2, when I try

Solve[{x + y == 12, y == 2 x}, {x}]

I get

{{x -> 4}}

Looks good. But, if I do

Solve[{x + y == 25, y == 4 x}, {x}]

I get

{{ }}

?

8 Replies

Looks like you accidentally wrote y = 2x instead of y == 2x at some point. Execute Clear[y] and try Solve again.

Thanks, Michael.

That works in mathematica online!

In[1]:=Solve[{x+y==25,y==4x},{x},{y}]
Out[1]={{x->5}}

Curiously, when I do this in 11.2, I get:

Solve::ivar: 2 x is not a valid variable.
Solve[{3 x == 25, 2 x == 4 x}, {x}, {2 x}]

Weird.

You can use the old syntax, specifying a list of variables to eliminate in the third argument:

Solve[{x + y == 25, y == 4 x}, {x}, {y}]
(*  {{x -> 5}}  *)

Thanks, Neil. I structured this exercise based on the problem in my algebra book, which specifically asks for it this way:

Solve for x if y=4x and x+y=25

So for the set of values for y of {4x}, provide x. I'm not all that interested in y, and I really don't want to get the value for it.

Oh, well. Solving for {x,y} does work. I'll keep an eye on it. Thanks, all!

Is there a reason you don’t use Solve to solve your two equations with two unknowns (x and y)?

If you change {x} to {x,y} you will always get the complete answer for either set of equations.

Thanks Valeriu and Tanel. It's cool that the Reduce function works, sort of (I would expect given y==2, the result should be in terms of y==2). The biggest question I don't understand is why Solve for the first formula works, but not for the second, in all cases. I can repeat this in both online and the desktop version.

Only Reduce is giving values

In[7]:= Reduce[{x + y == 25, y == 4 x}, x]

Out[7]= y == 20 && x == 5

and

In[8]:= Reduce[{x + y == 12, y == 2 x}, {x}]

Out[8]= y == 8 && x == 4

I get for both of them:

In[1]:= Solve[{x + y == 12, y == 2 x}, {x}]
Out[1]= {}

In[2]:= Solve[{x + y == 25, y == 4 x}, {x}]
Out[2]= {}

As you try to solve systems with respect to only one variable, the results are correct.

Reply to this discussion
Community posts can be styled and formatted using the Markdown syntax.
Reply Preview
Attachments
Remove
or Discard

Group Abstract Group Abstract