36a^2-(9a^2+1)^2
=(6a)^2-(9a^2+1)^2
=[6a+(9a^2+1)][6a-(9a^2+1)]
=(9a^2+6a+1)[-(9a^2-6a+1)]
=(3a+1)^2×（-1）×(3a-1)^2
=-(3a+1)^2(3a-1)^2