W10,Sagemath 8.4 solve() does not give a response

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why Sagemath solve() does not give a response, and no indication on what it is missing ? (by hand a_1=a_2=3/15)

var('X_1 X_2 a_1 a_2 b Theta')
assume(a_1,'real');assume(a_2,'real')
assume(X_1,'real');assume(X_2,'real')
assume(b,'real') ;assume(Theta,'real')
f_1=2*X_1^2*a_1 + 2*X_1*X_2*a_2 - 2*Theta*X_1 + 2*X_1*b
f_2=2*X_1*X_2*a_1 + 2*X_2^2*a_2 - 2*Theta*X_2 + 2*X_2*b
S=solve ([f_1==0,f_2==0,X_1*X_2==1,Theta*X_1==1,Theta*X_2==1,X_1*X_1==4,X_2*X_2==4,Theta*Theta==4,b==0],a_1,a_2)
show(S)