Draw two parallel lines PQ, MN. Construct two triangles ABC and ABD with AB a part of line MN and C and D both points on line PQ .

Draw AR parallel to CB such that R lies on PQ

Draw BS parallel to AD such that S lies on PQ

ARCB is a parallelogram         (Opp. sides parallel) 

ADSB is a parallelogram         (Opp. sides parallel)

ARCB and ADSB have equal areas  (Parallelograms on Same Base are equal)

Tri. ABC = Half of area ABCR    (Parallelogram diag. creates equal tri.s) 

         = Half of area ADSB    (Shown above)

         = Tri. ABD             (Parallelogram diag. creates equal tri.s)