[Maxima] in sector , if A=S, solve r=?

[=?big5?B?tsAguM62rw==?=]

A:1/2*S*r;
solve([A=S],[r]);

(%o6) (r*S)/2
(%o7) [r=2]
================================
how to solve if some variable is constant
> we have
> Q is a angle of sector
> r is a radius of sector
> S is a arc length of sector
> A is a area of sector
> 
> question : if the value of A = the value of S, solve r=?
> that is clearly, r=2.
> A=S
> 1/2Sr=S
> so, r=2
> 
> I try to do  in maxima
> A:1/2*S*r;
> solve([A=S,r]);
> solve([1/2*S*r=S,r]);
> 
> (%o13) (r*S)/2
> (%o14) [[S=0,r=0]]
> (%o15) [[S=0,r=0]]
> that can not  answer r=2.

 		 	   		  
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