|Re: Need help evaluating results of implicit differentiation.|
Message #2 Posted by Crawl on 7 Oct 2010, 12:23 p.m.,
in response to message #1 by Hal Bitton in Boise
It seems like the easiest thing to do would be to go into Equation Editor, select Y(X) with the cursor, and replace it with 3.
However, I can imagine situations where that wouldn't work (if there are a lot of Y(X)'s in the result and it would be tedious to replace them all by hand, or if you wanted to automate it with a program).
So here's another way. When you have your expression, enter 'Y(X)' on the stack. Hit DERVX so you have d1Y(X) on the stack (or just enter 'd1Y(X)' to begin with). Then enter some other variable that you're not using, like Z. Then =, SUBST.
Now you can enter 'Y(X)', 3, =, SUBST, and it'll substitute in, leaving the derivative alone (otherwise, it thinks you're saying Y(x) is the constant 3, so its derivative is 0).
You just have to solve for Z now rather than d1Y(X). Or if you really want it back the other way, 'Z=d1Y(X)' SUBST at the end also works.