#1
(x-1)(x+1)+x²=x²-1+x²=2x²-1
#2
a(a-b)(a+b)=a(a²-b²)=a³-ab²
#3
(4-3y)(4+3y)+18y²=16-9y²+18y²=16+9y²
#4
x(2x-z)(2x+z)+4xz²=z(4x²-z²)+4xz²=4x²z-z³+4xz²
#5
(2b-5c)(2b+5c)+(b-c)(b+c)=4b²-25c²+b²-c²=5b²-26c²
#6
(3x-y)(3x+y)+(x-2y)(x+2y)=9x²-y²+x²-4y²=10x²-5y²
#7
(x²-1)(x²+1)+x⁴=x⁴-1+x⁴=2x⁴-1
#8
(x³-y)(x³+y)+(y-2x³)(y+2x³)=x⁶-y²+y²-4x⁶=-3x⁶