1)(m+n)(m-n)(m^2-n^2)+2m^2n^2 ; 2)(a+3)(a-3)(a-3)(a+3)+18a^2 ; 3)(x+2)^2 × (x-2)^2-x^4+16

1)(m+n)(m-n)(m^2-n^2)+2m^2n^2=(m^2-n^2)(m^2-n^2)+2m^2n^2=(m^2-n^2)^2+2m^2n^2=m^4-2m^2n^2+n^4+2m^2n^2=m^4+n^4
2)(a+3)(a-3)(a-3)(a+3)+18a^2=(a^2-9)(a-3)(a+3)+18a^2=(a^2-9)(a^2-9)+18a^2=(a^2-9)^2+18a^2=a^4-18a^2+81+18a^2=a^4+81
3)(x+2)^2 × (x-2)^2-x^4+16=((x+2)(x-2))^2-x^4+16=(x^2-4)^2-x^4+16=x^4-8x^2+16-x^4+16=-8x^2+32