`i) 64x^4+81`
`=64x^4+144x^2+81-144x^2`
`=(8x^2)^2+2.8x^2.9+9^2-(12x)^2`
`=(8x^2+9)^2-(12x)^2`
`=(8x^2+12x+9)(8x^2-12x+9)`
`k) x^8+4y^4`
`=x^8+4x^4y^2+4y^4-4x^4y^2`
`=(x^4+2y^2)^2-(2x^2y)^2`
`=(x^4-2x^2y+2y^2)(x^4+2x^2y+2y^2)`
`h) x^8+x^7+1`
`=x^8-x^2+x^7-x+x^2+x+1`
`=x^2(x^6-1)+x(x^6-1)+(x^2+x+1)`
`=x^2(x^3-1)(x^3+1)+x(x^3-1)(x^3+1)+(x^2+x+1)`
`=(x-1)(x^2+x+1)(x^5+x^2)+(x-1)(x^2+x+1)(x^4+x)+(x^2+x+1)`
`=(x^2+x+1)[(x-1)(x^5+x^2)+(x-1)(x^4+x)+1]`
`=(x^2+x+1)(x^6+x^3-x^5-x^2+x^5+x^2-x^4-x+1)`
`=(x^2+x+1)(x^6-x^4+x^3-x+1)`