In the given figure, quadrilateral ABCD circumscribes a circle, as shown. Then, AB + CD =
AD + BC
We know tangents from an external point are equal.
AP = AS .... (i)
PB = BQ .... (ii)
CR = QC .... (iii)
RD = SD .... (iv)
Adding we get, (AB + PB) + (CR + RD) = (AS + SD) + (BQ + QC)
AB + CD = AD + BC