11. A rectangular shaft has a cross section with a width of 
and
a thickness of 
shown in the figure. It is designed
to transimit a torque 
. The shear modulus is 
. If the allowable angle of twist
is 
and the proability of failure is
designed to be 
, determined the maximum length
of the shaft. Note that 
and 
are
independent.
Answer: 
, 
