Because ∠bea 1 =∠A+∠ age.

Because ∠ age =∠CGA 1

Because ∠ CGA1= ∠ A1+∠ CFA1.

so∠bea 1 =∠A+∠AGE =∠A+∠CGA 1 =∠A+∠A 1+∠CFA 1。

Because ∠A=∠A 1

So ∠BEA 1=2∠A+∠CFA 1.

That is, ∠ bea1-∠ CFA1= 2 ∠ a.