`a)4x^4 + 1`
`= (2x^2)^2 + 4x^2 + 1 - 4x^2`
`= (2x^2 + 1)^2 - (2x)^2`
`= (2x^2 +2x + 1)(2x^2 - 2x+1)`
`b) 4x^4 + y^4`
`= 4x^4 + 4x^2y^2 - 4x^2y^2 + y^4`
`= (2x^2 + y^2)^2 - (2xy)^2`
`= (2x^2 + y^2 -2xy)(2x^2 +y^2 + 2xy)`
`c) x^4 + 324`
`= x^4 + 36x^2 + 324 - 36x^2`
`= (x^2 + 18)^2 - 36x^2`
`= (x^2 + 18 - 6x)(x^2 + 18 + 6x)`