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I have two equations as below:

eq1: $8 \pi \gamma r-\frac{4 \pi \text{Delta$\_\mu $} r^2}{\text{v$\_$c}}=0$

eq2: $\text{Delta$\_\mu $}=\text{K$\_$b} S T$

When I try Solve[{eq1}, {r}], I obtained $\left\{\{r\to 0\},\left\{r\to \frac{2 \gamma \text{v$\_$c}}{\text{Delta$\_\mu $}}\right\}\right\}$.

This is correct, but I want it is expressed as $\left\{r\to \frac{2 \gamma \text{v$\_$c}}{\text{K$\_$b} S T}\right\}$. I tried Solve[{eq1, eq2}, {r}] but I obtained {}

How can I get the result expressed with K_b, S and T instead of Delta_u? Many thanks!

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    $\begingroup$ it is better to post the actual Mathematica plain source code you used. Also _ can't be used in variable names. $\endgroup$ – Nasser Dec 21 '15 at 11:03
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    $\begingroup$ Try Eliminate. $\endgroup$ – Sjoerd C. de Vries Dec 21 '15 at 11:11
  • $\begingroup$ Or Solve for both Delta_u and r (I trust the actual "variable" does not have an underbar in it). $\endgroup$ – Daniel Lichtblau Dec 21 '15 at 16:10
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Format[v[c]] := Subscript[v, c];
Format[K[b]] := Subscript[K, b];
Format[Delta[u]] := Subscript[Delta, u];

Solve[8 π γ r - 4 π Delta[u] r^2/v[c] == 0, r] /. 
 Delta[u] -> K[b] S T

enter image description here

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  • $\begingroup$ Thanks! This is exactly what I want. Sorry that I cannot vote for your answer since I do not have sufficient reputation. $\endgroup$ – user2230101 Dec 22 '15 at 6:18

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