Question 662515
The line {{{ y = 5x }}} passes through the origin and
also passes through the 3rd quadrant. If I say this line
has a slope of {{{ 5 }}} and makes an angle {{{ a }}}
and {{{ tan( a ) = 5 }}}
with the +x axis, then the rotation of the line is
{{{ pi + a = theta }}} 
{{{ sin( theta ) = -5 / sqrt( 26 ) }}}
{{{ cos( theta ) = -1 / sqrt( 26 ) }}}