[(x-2y)(x+2y)]² +[(x-2y)² -(x+2y)²]²
= [x²-4y²]² +[x²-4xy+4y² -x²-4xy-4y²]²
= [x⁴-8x²y²+16y⁴] +16x²y²
= x⁴+8x²y²+16y⁴
= (x²+4y²)²
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