On 7/23/10 1:23 PM, William Porter wrote:
> Solve for y when [x=-10, x=10] and d=5. /x/ and /d/ are input
> parameters. /x/ will first be set a value and then /d/ is assigned a
> value for e1: x^2+d*y*4+d^2*(-4).
>
>
>
> I have the following expression:
>
> (%i1) e1: x^2+d*y*4+d^2*(-4);
>
> 2 2
>
> (%o1) 4 d y + x - 4 d
>
> (%i2) e2: solve(e1, y);
>
> 2 2
>
> x - 4 d
>
> (%o2) [y = - ---------]
>
> 4 d
>
> I know that ?
>
> (%i3) e3: subst([y=0, d=5], e1);
>
> 2
>
> (%o3) x - 100
>
> (%i4) e4: solve(e3, x);
>
> (%o4) [x = - 10, x = 10]
>
>
>
> (%o3) and (%o4) lines are correct. But I need to assign a value to /x/
> for example /x/=[-10,10] and/ d:5 /to determine /y/ value. I know that
> the answer is y=0. Another example is /x/=[-50,50] and /d/:50. I know
> that the answer is /y/=37.5. The problem is I am lost as how to solve
> this issue.
I don't really understand the issue, but what's wrong with
subst([x=50, d=50], e2)
[y= 75/2]
Ray