I didn't understand the video , please explain me

Hi!

Here is the answer to your question.

 

The measure of exterior angle of a triangle is equal to sum of its two interior opposite angles.

 

 

Here, ∠ABD is the exterior of ∆ABC and BF||AC.

 

BF || AC and AB is the transversal,

∠ABE = ∠CAB     … (1)    (Pair of alternate interior angles)

 

BF || AC and CD is the transversal,

∠DBE = ∠BCA    … (2)    (Corresponding angles)

 

Adding (1) and (2), we get

∠ABE + ∠DBE = ∠BCA + ∠CAB

From the given figure, we have ∠ABE + ∠DBE = ∠ABD

∴ ∠ABD = ∠BCA + ∠CAB

Thus, the measure of exterior angle of a triangle is equal to sum of its two interior opposite angles.

 

Cheers!