Derivation of reflection and transmission probability formulas

[MaxJ]

Homework Statement

below

Relevant Equations

below

For this,

The solution is

I have doubt where they got the reflection probability formula from. Someone may know how to find it. I think that $R + T = 1$. But I'm not sure where the transmission probability formula comes from either.

Kind wishes

 

[Orodruin]

Did you try to do what they actually say in the text?

 

[MaxJ]

Did you try to do what they actually say in the text?

View attachment 349853

Sir, blessed.

I have a doubt where to start in the proof. Not sure what logic to use.