(1)a³-4a
=a(a²-4)
=a(a+2)(a-2)
(2)(x²+y²)²-4x²y²
=x^4+2x²y²+y^4-4x²y²
=x^4-2x²y²+y^4
=(x²-y²)²
(3)(a+b)²-10a-10b+25
=(a+b)²-10(a+b)+25
=(a+b-5)²