What I really want is sage to give me the answer as:
q1 = 0.58239...(however many digits)*q2
Is there a way to do that? I don't want an approximate value of the
entire thing (obviously you can't get that without giving the value of
q2), but I want it to compute whatever it can (in this case that
(223317 pi + 416000)/(223317 pi + 416000) = 0.58239...
Note:
An error in my first post:
When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
((223317 pi + 416000) q2)*q2
SHOULD BE:
When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
(223317 pi + 416000)*q2
What I want:
q1 = 0.58239...*q2
On Oct 15, 5:16 pm, kcrisman <kcris...@gmail.com> wrote:
> Dear Scott,
>
> Thanks for posting.
>
> On Oct 15, 6:46 pm, Scott <spectre...@gmail.com> wrote:
>
>
>
> > I know this is really basic, but for some reason I cannot figure this
> > out and my searching (here, sage website, google, etc) has not yielded
> > me any results! I just started using sage, literally, yesterday.
>
> > I am using solve() to solve an equation for q1 in terms of q2. The
> > solution of this is saved, but when I print it, it prints as an exact
> > answer, but I want it in decimal/approximate form. I have tried using
> > the n() function, but I get an error when trying it.
>
> > Here is an example:
> > --------------------------------------------------
> > q1, q2 = var('q1, q2')
> > r = 0.60
> > t1 = 0.005
> > t2 = 0.007
> > d = 2.0
> > A1 = (1/2)*pi*r*r
> > A2 = (1/2)*d*(2*r)
> > theta1 = 1/(2*A1)*(q1*2*pi*r/t1+(q1-q2)*2*r/t1)
> > theta2 = 1/(2*A2)*(q2*d/t2+q2*sqrt(d^2+(2*r)^2)/t2+(q2-q1)*2*r/t1)
> > sol_q1 = solve([theta1==theta2], q1)
> > print sol_q1
> > print sol_q1[0].substitute(q2=1).right().n()
> > --------------------------------------------------
>
> > When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
> > ((223317 pi + 416000) q2)*q2
> > The last line does work to give me the desired value (0.58239....),
> > but this is definitely not an ideal way of getting it! And of course
> > this wouldn't work for a more complicated result (since I use q2=1).
>
> Why not? You should be able to put whatever you want in for q2.
> Maybe I am misunderstanding the question.
>
> > I've tried many other combinations of sol_q1.n(), sol_q1[0].n(), etc,
>
> All these will do nothing if you haven't specified q2, because you are
> trying to approximate something which has no approximation; you have
> to substitute all free variables before approximating. I suppose it
> would be possible to make Sage do everything on coefficients, but I
> don't think that is the usual functionality. Your solution is
> actually pretty good; even doing .substitute(q2=1.0) wouldn't help,
> because we prefer to leave pi unapproximated until necessary. I do
> agree that it's annoying! But I'm not sure how to make it less so,
> short of writing another function that evaluated a symbolic equation
> at a given point - without checking that you had something like
>
> sage: solve([sin(x-y)==x*y],y)
> [y == -sin(-x+y)/x]
>
> where it would cause trouble.
>
> Hope this helps!
>
> - kcrisman
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