Wrong solution?

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Hi, could you help me with this solution of two equations on the interval:

x,a=var('x,a')

f1=10/x

f2=x-5*a

assume(a>0,x>0)

print(solve([f1==f2],x))

Solution given by Sage is:

[ x == 5/2a - 1/2sqrt(25*a^2 + 40),

x == 5/2a + 1/2sqrt(25*a^2 + 40) ]

The first solution is obviously not solution for me as it is always strictly negative and I don't understand why the Sage gave me it. Assumptions are clear: x has to be >0 When I change second equation slightly:

x,a=var('x,a')

f1=10/x

f2=x-6*a

assume(a>0,x>0)

print(solve([f1==f2],x))

Solution given by Sage is correct now (only the x>0 are reported):

[ x == 3a + sqrt(9a^2 + 10) ]

Could you help me explain the difference in results. I am not sure if problem is on python side or with some rules how Sage computes the results. Thank you in advance.