FOR THE 3RD QUESTION CAN WE USE THE METHOD SHOWN IN THE 2ND VIDEO

In the 3^rd example we apply the angle sum property of triangle which is explained in the second video of this lesson.

This property states that the sum of the angles of a triangle is 180º.

The given figure shows ΔABC, in which BX and CY bisect ∠ABC and ∠ACB respectively. BX and CY intersect at point O. Prove that

Applying angle sum property in ΔOBC, we obtain

∠BOC + ∠OBC + ∠OCB = 180°

⇒ ∠BOC = 180° – (∠OBC + ∠OCB) … (1)

Now, BX and CY are the bisectors of ∠ABC and ∠ACB respectively. So, we will use this information to get the required result.

∴ ∠OBC = ∠ABC … (2)

Similarly,  ∠OCB = ∠ACB … (3)

Using equation (2) and (3) in equation (1), we obtain

Hope you will get it now!!