Ratio from a symmetric condition

Optional extension

Assume \(a-b\ne0\), so the given ratio is defined.

Given \(a^2+b^2=4ab\):

1. expand and simplify \((a-b)^2\)[2]
2. if \(\frac{a+b}{a-b}=k\) and \(k>0\), find \(k\).[3]