|a+4| ≥ 0,b^2-2b+1 = (b-1)^2 ≥ 0
两个互为相反数,所以|a+4| = b^2-2b+1 = 0
所以a = -4,b = 1
（x^2+4y^2)-(-4xy + 1)
= x^2+4xy+4y^2 - 1
= (x+2y)^2 -1
= (x+2y+1)(x+2y-1)