~~Is this wrong ~~Wrong solution?

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

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

f1=10/x

~~f2=x-5*a~~

f2=x-**5***a

**assume(a>0,x>0)**

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

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

Solution given by Sage is:

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

x == 5/2*a + 1/2*sqrt(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)))**

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

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

[
x == 3*a + sqrt(9*a^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.