11.9x^4-13x^2y^2+4y^4
=（x^2-y^2)（9x^2-4y^2）
=(x-y)(x+y)(3x-y)(3x+y)
12.x^(n+1)(x^4-7x^2-8)
=x^(n+1)(x^2+1)(x^2-8)
13.原式=(a^2+a+1)(a+1)+2
=2
14.原式=(a^3+b^3)+3ab
=(a+b)(a^2+b^2-ab)+3ab
=a^2+b^2+2ab
=(a+b)^2
=1
15.设(a-1)(a+m)=a^2+5a+k
则-1m=k
m-1=5
解得m=6,k=-6