Explanation:

Let say I'm using javascript as a programming language:

1. 3xy

In javascript: 3 * x * y

2. x2 + y2

In javascript: (x * x) + (y * y)

3. 3x/y3

In javascript: (3 * x) / (y * y * y)

4. a2 + b2 / cd

In javascript: ((a * a) + (b * b)) / (c * d)