1)(x-1)(x+1)+x^2=x^2-1+x^2=2x^2-1
2)a(a-b)(a+b)=a(a^2-b^2)=a^3-b^2a
3)(4-3y)(4+3y)+18y^2=16-9y^2+18y^2=9y^2+16
4)x(2x+z)(2x-z)+4xz^2=x(4x^2-z^2)+4xz^2=4x^3-xz^2+4xz^2=4x^2+3xz^2
5)(2b-5c)(2b+5c)+(b-c)(b+c)=4b^2-25c^2+b^2-c^2=5b^2-26c^2
6)(3x-y)(3x+y)+(x-2y)(x+2y)=9x^2-y^2+x^2-4y^2=10x^2-5y^2