how to rewrite an equation in terms of another equation?

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Omidreza Ghafarinejad
评论： Star Strider 2015-4-26
Hi. I have an equation for example like this:
sigma = 4*q*y^3/(b*h^3)
and I introduce that:
I = b*h^3/12
now I want to know how can I rewrite "sigma" in terms of "I"?
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Omidreza Ghafarinejad
Omidreza Ghafarinejad 2015-4-26
编辑：Omidreza Ghafarinejad 2015-4-26
The question is very simple. I want to rewrite sigma in terms of I. I don't want to do it by hand. I want matlab to do this job for me. Consider that the sigma here is just an example. I have 3 equations that are longer than sigma here.Calculating all of them in terms of "I" takes so much time so I want matlab to do it for me. For example it should show me the sigma like this:
sigma = q*y^3/3*I
pfb
pfb 2015-4-26
Ok, now that you have explained it, I understand. Perhaps I'm a bit slow.
Anyway, duplicating the question in the "tags" field is not very helpful.
Referring to symbolic calculations would have been more helpful.

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采纳的回答

Star Strider
Star Strider 2015-4-26
This works:
syms q y b h I
sigma = 4*q*y^3/(b*h^3);
Eq1 = I == b*h^3/12;
b = solve(Eq1,b);
sigma = subs(sigma)
produces:
sigma =
(q*y^3)/(3*I)

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