math(geometry)
posted by Hoema .
find the altitude on hypotenuse if a=6cm and b=8cm?

I'm guessing that you mean the hypotenuse is 8 cm.
a^2 + c^2 = b^2
6^2 + c^2 = 8^2
36 + c^2 = 64
c^2 = 64  36
c^2 = 28
c = 5.2915
If you need to find the hypotnuse  then
6^2 + 8^2 = c^2
36 + 64 = c^2
100 = c^2
10 = c
The second solution is probably the one you need. 
I think I understand now. You are looking for the LENGTH of the hypotenuse.

or, if D is the foot of the altitude on the hypotenuse AB, then by similar triangles
CD/CA = CB/AB
CD/6 = 8/10
CD = 48/10 = 4.8