Wave intensity at different points

[desmond iking]

Homework Statement

here's the question , ans any my hand written working. which one is correct ? my ans or the model answer?

Homework Equations

The Attempt at a Solution

 

[rude man]

Theirs is.
Why are you introducing r? They are giving you the amplitudes already, you don't have to calculate them. And intensity is proportional to amplitude squared. This is true for water waves and it's true for electromagnetic waves.