`4)`
`a)P=(3x+4)^2-10x-(x-4)(x+4)`
`P=(9x^2+24x+16)-10x-(x^2-16)`
`P=9x^2+24x+16-10x-x^2+16`
`P=8x^2+14x+32`
`b)A=(4x^2+y^2)(2x+y)(2x-y)`
`A=(4x^2+y^2)(4x^2-y^2)`
`A=16x^4+y^4`
`5)(7n-2)^2-(2n-7)^2`
`=(49n^2-28n+4)-(4n^2-28n+49)`
`=49n^2-28n+4-4n^2+28n-49`
`=45n^2-45`
`=9(5n^2-5)vdots9`
`(7n-2)^2-(2n-7)^2vdots9∀n,ninZZ`