Factorise `16x^(4)-81y^(4)`.

Explanation

To factorize the expression \( 16x^4 - 81y^4 \), we can follow these steps:

### Step 1: Identify the expression as a difference of squares
We notice that \( 16x^4 \) and \( 81y^4 \) can be expressed as squares:
\[ 16x^4 = (4x^2)^2 \quad \text{and} \quad 81y^4 = (9y^2)^2 \]
Thus, we can rewrite the expression as:
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