Here's one way to do this, tertre.....but maybe not the most elegant !!!!
Let B = (0,0) C = (8,0)
Since AM is a median drawn to BC....then M is (4,0)
Now.....construct a circle with a radius of 5 centered at the origin
The equation of this circle is
x^2 + y^2 = 25 (1)
And construct a cirrcle with a radius of 4 centered at M
The equation of this circle is
(x - 4)^2 + y^2 = 16 (2)
Subtract (2) from (1) and we have that
x^2 - (x - 4)^2 = 9 simplify
x^2 - x^2 + 8x - 16 = 9
8x = 25
x = 25/8
This is the x coordinate of A
To find the y coordinate, we have
(25/8)^2 + y^2 = 25
625/64 + y^2 = 25
y^2 = 25 - 625/64
y^2 = 1600/64 - 625/64
y^2 = 975/64 take th positive root
y = sqrt (975)/ 8
So A = (25/8, sqrt (975/8)
So....using the distance formula AC =
sqrt [ (8 - 25/8 )*2 + 975/64 ] =
sqrt [ ( 39^2) / 64 + 975 / 64 ] =
sqrt (39^2 + 975) / 8 =
sqrt (2496) / 8 =
sqrt (2^6 * 3 * 13) / 8 =
sqrt (64 * 3 * 13 ) / 8
8sqrt (39) / 8
sqrt (39)
Here's a pic :