-5a^2+25a
=-5a(a-5)
-36x^2+12xy-y^2
=-[(6x)^2-2×6xy+y^2]
=-(6x-y)^2
9（a+b)^2-4（a-b)^2
=[3(a+b)]^2-[2(a-b)]^2
=(3a+3b+2a-2b)(3a+3b-2a+2b)
=(5a+b)(a+5b)