27) `4x^4-16x^2y^2+16y^4 = (2x^2)^2 - 2.2x^2.4y^2 + (4y^2)^2 = (2x^2+4y^2)^2`
28) `9x^4 - 12x^5 + 4x^6 = (3x^2)^2 - 2.3x^2.2x^3 + (2x^3)^2 = (3x^2 - 2x^3)^2`
29) `x^10 - 4x^8 +4x^6 = (x^5)^2 - 2.x^5 . 2x^3 + (2x^3)^2 = (x^5+2x^3)^2`
30) `8x^6 - 27y^3 = (2x^2)^3 - (3y)^3 = (2x^2-3y)(4x^3+6x^2y+9y^2)`
31) `(a+b)^3-c^3 = (a+b-c)(a^2+2ab+b^2 + ac+cb + c^2)`