Let x = AC and y = BC and AB is the hypotenuse
CM = y/2
CN = x/2
Triangles AMC and BNC are both right
AC^2 + ( BC/2) = AM
(AC/2)^2 + BC^2 = BN
So.....by the Pythagorean Theorem we have these two equations
x^2 + (y/2)^2 =1
(x/2)^2 + y^2 = 1 simplify
x^2 + y^2/4 =1 → 4x^2 + y^2 = 4 (1)
x^2/4 + y^2 =1 → -x^2 - 4y^2 = -4
Add the last two equations and we get that
3x^2 - 3y^2 = 0
3x^2 = 3y^2
x^2 = y^2
Sub this back into (1) for y^2
4x^2 + x^2 = 4
5x^2 = 4
x^2 = 4/5
and
y^2 = 4/5
AC^2 + BC^2 = AB^2
4/5 + 4/5 = AB^2
AB = sqrt [ 4/5 + 4/5 ]
AB = sqrt [ 8/5] = 2sqrt [2/5] = (2/5)sqrt 10