If l, m, n are three parallel lines and the transversals t1 and t2 cut the lines l, m, n at the points A, B, C and P, Q, R as shown in the figure, then:
Drop perpendiculars ADE and PST
From the similar triangles ABD and ACE
ABAC=ADAE.......(i)
Also from the similar triangles PSQ and PTB
PQPR=PSPT.......(ii)
Since AD=PS and AE=PT
ABAC=PQPR
Applying dividendo
ABAC−AB=PQPR−PQABBC=PQQR