Three to five. Solution: ∫D is the midpoint of BC, BC= 10cm,

∴DC=BD=5cm，

∫BD2+AD2 = 144+25 = 169，AB2= 169

∴BD2+AD2=AB2，

∴△ABD is a right triangle, and∠ ∠ADB = 90°.

∴△ADC is also a right triangle and AC is the hypotenuse.

∴AC2=AD2+DC2=AB2

∴AC= 13cm.

So the answer is: 13.

80.4 is ... the root number is 3 squared, and then it will be fine.