(1).4ab-2a^2-2b^2
=-2(a^2-2ab+b^2)
=-2(a-b)^2
(2).-8ax^2+16axy-8ay^2
=-8a(x^2-2xy+y^2)
=-8(x-y)^2
(3).5.9^2-2*5.9*0.9+0.9^2
=(5.9-0.9)^2
=5^2
=25