a)
x²-2x+1-y²=(x-1)²-y²
b)
x²-4y²+6x+9=x²+6x+9-4y²=(x+3)²-4y
c)
16x²-16y²-8x+1=(4x-4y)(4x+4y)-8x+1
e)
x²-y²-4yz+4z²=x²-(y-2z)²
f)
x^4-25+2x³+x²=(x²-5)(x²+5)+x²(2x+1)
g)
(x²+y²-z²)²-4x²y²=x^4+y^4+z^4+2x²y²-2x²z²-2y²z²-4x²y²
=x^4+y^4+z^4-2x²y²-2x²z²-2y²z²
=(x²-y²)²+z^4-2(x²y²+y²z²)
h)
25a²-4b²+12bc-9c²=(5a-2b)(5a+2b)+12bc-9c