Question 1130301
Here's the plot:
{{{ graph( 600, 400, -12, 12, -20, 20, 3*( 1/cos( x/2 ))) }}}
It looks like one period of {{{ x/2 }}} would be {{{ -pi/2}}} to {{{ 3pi/2 }}}
so then: 
{{{ x/2 = -pi/2 }}}
{{{ x = -pi }}}
and
{{{ x/2 = 3pi/2 }}}
{{{ x = 3pi }}}
which is what I see
The asymptotes are at {{{ pi }}} and {{{ -pi }}}