If line ab has a slope of -1/2 and is perpendicular to line cd, then the slope of line cd is

Take the opposite recporical of the slope give...so the so that would be. 2.

when two lines are perpendicular, switch the numerator and denominator of the given slope, add a negative sign in the front, you will have the slope of the other line. 
if the slope is an integer, for example 3, it is the same as 3/1, the slope of the perpendicular line is then negative 1/3, (-1/3)
in this case, the slope of CD is -(-2/1)=2