The reasons are as follows: set the flagpole height ab = X.
If F is passed, FG⊥AB is in G and CE is in H (as shown in the figure).
∴△AGF∽△EHF.
∫FD = 1.5,GF=27+3=30,HF=3,
∴eh=3.5﹣ 1.5=2,ag=x﹣ 1.5.
From △AGF∽△EHF, we get: =,
That is =,
∴x﹣ 1.5=20,
Solution: x=2 1.5 (m).
Answer: The height of the flagpole is 21.5m. 。