Giải thích các bước giải:
`a, \ x^2-9=x^2-3^2`
`=(x-3)(x+3)`
`b, \ (x+1)^2-9`
`=(x+1)^2-3^2`
`=(x+1-3)(x+1+3)`
`=(x-2)(x+4)`
`c, \ (4x-1)^2-9x^2`
`=(4x-1)^2-(3x)^2`
`=(4x-1-3x)(4x-1+3x)`
`=(x-1)(7x-1)`
`d, \ (x+2)^2-(3x-1)^2`
`=(x+2-3x+1)(x+2+3x-1)`
`=(3-2x)(4x+1)`
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Áp dụng : `A^2-B^2=(A-B)(A+B)`