简单题！！！！！火速给分！！！！！！！！！

①16m^2+16=16(m^2+1)
②x^2+2xy-y^2=x^2+2xy+y^2-2y^2=(x+y)^2-(√2y)^2=(x+y+√2y)(x+y-√2y)=[x+(1+√2)y][x+(1-√2)y]
③t^2+t+1/4=(t+1/2)^2
④2008x^2-2008b^2=2008(x+b)(x-b)
⑤x^2+10xy+25y^2=(x+5y)^2
⑥x^2-5x+6=(x-2)(x-3)
⑦a^2-2a-3=(a+1)(a-3)

下列各式中，不能分解因式的有几个（1个）【要原因】我知道答案
①16m^2+16; ②x^2+2xy-y^2; ③t^2+t+1/4; ④2008x^2-2008b^2; ⑤x^2+10xy+25y^2; ⑥x^2-5x+6; ⑦a^2-2a-3
②x^2+2xy-y^2不能因式分解
16m^2+16=16(m^2+1)
t^2+t+1/4=(t+1/2)^2
2008x^2-2008b^2
=2008(x^2-b^2)
=2008(x+b)(x-b)
x^2+10xy+25y^2=(x+5y)^2
x^2-5x+6=(x-2)(x-3)
a^2-2a-3=(a-3)(a+1)