Question 1099978
{{{ cos( A ) = -4/5 }}}
The angle is in the 2nd quadrant
cos is negative
sin is positive 
tan is negative
{{{ tan( A/2 ) = ( 1-cos(A) ) / sin( A ) }}}
{{{ tan( A/2 ) = ( 1 -(-4/5) ) / ( 3/5 ) }}}
{{{ tan( A/2 ) = ( 5/5 + 4/5 ) / (3/5) }}}
{{{ tan( A/2 ) = ( 5 + 4 )/3 }}}
{{{ tan( A/2 ) = 3 }}}
( note that {{{ A/2 }}} is in the 1st quadrant )