(x²-y+1)(x²+1)+x²y+y-x²
=(x²-y+1)(x²+1)+y(x²+1)-x²
=(x²+1)(x²-y+1+y)-x²
=(x²+1)²-x²
=(x²+1+x)(x²+1-x)
(x+5y)²+(2x+10y)(3x-y)+(y-3x)²
=(x+5y)²-2(x+5y)(y-3x)+(y-3x)²
=(x+5y-y+3x)²
=(4x+4y)²
=16(x+y)²