`16) 1 - 8x + 16x^2 - y^2`

`= (1 - 4x)^2 - y^2`

`= (1 - 4x - y)(1 - 4x + y)`

`17) x^2 - 16y^2 + 4x + 4`

`= (x^2 + 4x + 4) - 16y^2`

`= (x + 2)^2 - (4y)^2`

`= (x + 2 - 4y)(x + 2 + 4y)`

`19) y^2 - 14y - 25x^2 + 49`

`= (y^2 - 14y + 49) - 25x^2`

`= (y - 7)^2 - (5x)^2`

`= (y - 7 - 5x)(y - 7 + 5x)`

`20) x^2 - 4y^2 + 16x + 64`

`= (x^2 + 16x + 64) - 4y^2`

`= (x + 8)^2 - (2y)^2`

`= (x + 8 - 2y)(x + 8 + 2y)`