>> x1 = [ 0, 7, 9 ];
>> y1 = [ 6, 1, 3 ];
>> [ x2, y2 ] = triangle_shrink ( x1, y1, 0.33 );
>> fill ( x1, y1, 'b' )
>> hold on
>> fill ( x2, y2, 'r' )
>> hold off
>> print ( '-dpng', 'hw039.png' )