Question: Imposing conditions on an expression while plotting

Hi,

restart:with(plots):

L1 := (1/2)*(S+sqrt(S^2+4*a*b))/b;

L2 := (1/2)*(S-sqrt(S^2+4*a*b))/b;

solve(L1=L2,S);

         2*sqrt(-a*b), -2*sqrt(-a*b)

solve(L1=L2,a);

             -(1/4)*S^2/b

solve(L1=L2,b);

                -(1/4)*S^2/a

Here is my question. 

Now plotting L1, L2 

a:=1:b:=1: # say

plot({L1,L2},S=0..1);

Is there a way to utilize the conditions which we can get through solve and use it while plotting (not 3d plotting) for any given choice of the values of a, b, S? 

 

 

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