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

Explanation

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

### Step 1: Rewrite the expression
We can express \( 16x^4 \) and \( 81y^4 \) as squares:
- \( 16 = 4^2 \)
- \( x^4 = (x^2)^2 \)
- \( 81 = 9^2 \)
- \( y^4 = (y^2)^2 \)
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