State with reasons, which of the following pairs of terms are of like terms and which are of unlike terms : (i) `7x, 12y` (ii) `15x-21x` (iii) `4ab,7ba`

Explanation

To determine whether the given pairs of terms are like terms or unlike terms, we need to analyze the variables in each pair. Like terms are those that have the same variables raised to the same powers, while unlike terms have different variables or different powers.

Let's go through the pairs step by step:

### Step 1: Analyze the first pair - `7x` and `12y`
- **Variables**: The first term has the variable `x`, and the second term has the variable `y`.
- **Conclusion**: Since `x` and `y` are different variables, `7x` and `12y` are **unlike terms**.

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