ΔAEF &
ΔACD are similar triangle. Hence their sides are in proportion.
AEAF=ACAD⇒AF=AE.ADAC __(1)
ΔADE & ΔABC are also similar
ADAB=DEBC⇒AD=DE.ABBC __(2)
AD×AD=AF×ACAE×DE×ABBC
AD2=(AF×AB)×AC.DEAE.BC
But, ACBC=AEDE (similarity proportional)
∴AD2=AF×AB×1
AD2=AF×AB
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