Bđt thứ `3:`
`a^2-b^2=(a+b)(a-b)`
`a,` `(7x-4)^2-(2x+1)^2`
`=[(7x-4)+(2x+1)][(7x-4)-(2x+1)]`
`=(9x-3)(5x-5)`
`=3(3x-1).5(x-1)`
`=15(3x-1)(x-1)`
`b, 9(x+1)^2-1`
`=[3(x+1)]^2-1^2`
`=[3(x+1)+1][3(x+1)-1]`
`=(3x+4)(3x+2)`
`c,` `9(x+5)^2-(x-7)^2`
`=[3(x+5)]^2-(x-7)^2`
`=[3(x+5)+(x-7)][3(x+5)-(x-7)]`
`=(3x+15+x-7)(3x+15-x+7)`
`=(4x+8)(2x+22)`
`=4(x+2).2(x+22)`
`=6(x+2)(x+22)`
`d,` `49(y-4)^2-9(y+2)^2`
`=[7(y-4)]^2-[3(y+2)]^2`
`=[7(y-4)+3(y+2)][7(y-4)-3(y+2)]`
`=(7y-28+3y+6)(7y-28-3y-6)`
`=(10y-22)(4y-34)`
`=2(5y-11).2(2y-17)`
`=4(5y-11)(2y-17)`
