`a)(1+x²)^2-4x(1-x²)`
`=1+2x²+x^4-4x+4x³`
`=1+4x²-2x²+x^4-4x+4x³`
`=(x^4+4x³+4x²)-(2x²+4x)+1`
`=[(x^2)^2+2.x².2x+(2x)²]-2(x²+2x)+1`
`=(x²+2x)²-2(x²+2x)+1`
`=(x²+2x-1)²`
`b)(x²-8)²+36`
`=x^4-16x²+64+36`
`=x^4-16x²+100`
`=x^4+20x²-36x²+100`
`=(x^4+20x²+100)-36x²`
`=[(x^2)^2+2.x².10+10²]-(6x)²`
`=(x²+10)²-(6x)²`
`=(x²+6x+10)(x²-6x+10)`
`c)81x^4+4`
`=81x^4+4+36x²-36x²`
`=(81x^4+36x²+4)-36x²`
`=[(9x²)^2+2.9x².2+2²]-(6x)²`
`=(9x²+2)-(6x)²`
`=(9x²+6x+2)(9x²-6x+2)`
