I used
Insphere[SSSTriangle[5, 7, 9][[1]]]
and got
Sphere[{11/2, Sqrt[11]/2}, Sqrt[11]/2]
But, I cannot get the result when I used
Insphere[SSSTriangle[a, b, c][[1]]]
I only got
Insphere[{{0, 0}, {c, 0}, {(-a^2 + b^2 + c^2)/(2 c), Sqrt[(a + b - c) (a - b + c) (-a + b + c) (a + b + c)]/(2 c)}}]
How can I get the correct result?