So the square of FD = the square of FB =FB*FC.
So FA/FB=FC/FA, so just prove △FAB∽△FCA.
∠∠DAF =∠DAC+∠CAF∠BAD+∠ABF =∠ADF∠ADF =∠DAF∠BAD =∠DAC
∴∠CAF=∠ABF
≈AFC =≈BFA
∴△FAB∽△FCA
∴FA/FB=FC/FA
∴ the square of FA =FB*FC
∴ the square of FD =FB*FC
You should understand, right? You can ask again if you don't understand.