Solving equations for Variable/Variable

Question

Isaac Kambouris 2021-9-4

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评论： Isaac Kambouris 2021-9-6

I have a simple question but i have not been able to find a way to achive this and was looking for some help. For example if I have the following code

syms A B C D E
eqn = (A*B*C)/E == D;

Is it there a function that will allow me to solve for C/D = ?. I have tried using solve but this provides a struct variable that solve the equation for c and D aka (solves for C= and D=).

Thanks

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Answer 1

John D'Errico 2021-9-5

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No reason you cannot do this:

syms A B C D E

eqn = (A*B*C)/E == D;

syms CoverD

solve(subs(eqn,C,CoverD*D),CoverD)

ans =

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John D'Errico 2021-9-5

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Just look. Take it apart.

syms A B C D E
eqn = (A*B*C)/E == D;
syms CoverD;

Effectively, I define the variable CoverD as C/D. So if we accept that

CoverD = C/D

then multiply by D. Can you solve for C? I hope so.

So what does this substitution do?

subs(eqn,C,CoverD*D)

ans =

It replaces C with the product CoverD*D. Now what happens to D in that expression? It effectively goes away, as long as D is not zero. And if D was zero, then what would be the meaning of the ratio C/D anyway? So we can assume D is never zero in this context.

The final solve should now be clear.

Isaac Kambouris 2021-9-6