.计算（1）4(x+1)^2 - (2x+5)(2x-5)
=4x^2+8x+4-4x^2+25
=8x+29
5.分解因式：
(1) x^3 - 9x
=x(x^2-9)
=x(x+3)(x-3)
(2) 16x^4 -1
=(4x^2+1)(4x^2-1)
=(4x^2+1)(2x+1)(2x-1)
(3) 6xy^2 - 9x^2 y-y^3
=-y(9x^2-6xy+y^2)
=-y(3x-y)^2
(4) (2a-b)^2+8ab
=4a^2-4ab+b^2+8ab
=4a^2+4ab+b^2
=(2a+b)^2